modifiers: Support Control and Alt

Control and Alt are special in that they aren't expected to switch levels, and so don't need to change what characters are output.

Use in layouts by adding `modifier: Control` or `modifier: Alt` in place of `text: "foo"`.

The latching of the modifier will force the keyboard to emit raw key presses and prevent it from outputting text.

src/data.rs

@@ -240,14 +240,20 @@ type ButtonIds = String;
 #[derive(Debug, Default, Deserialize, PartialEq)]
 #[serde(deny_unknown_fields)]
 struct ButtonMeta {
-    /// Special action to perform on activation. Conflicts with keysym, text.
+    // TODO: structure (action, keysym, text, modifier) as an enum
+    // to detect conflicts and missing values at compile time
+    /// Special action to perform on activation.
+    /// Conflicts with keysym, text, modifier.
     action: Option<Action>,
     /// The name of the XKB keysym to emit on activation.
-    /// Conflicts with action, text
+    /// Conflicts with action, text, modifier.
     keysym: Option<String>,
     /// The text to submit on activation. Will be derived from ID if not present
-    /// Conflicts with action, keysym
+    /// Conflicts with action, keysym, modifier.
     text: Option<String>,
+    /// The modifier to apply while the key is locked
+    /// Conflicts with action, keysym, text
+    modifier: Option<Modifier>,
     /// If not present, will be derived from text or the button ID
     label: Option<String>,
     /// Conflicts with label
@@ -270,6 +276,20 @@ enum Action {
     Erase,
 }
 
+#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[serde(deny_unknown_fields)]
+enum Modifier {
+    Control,
+    Shift,
+    Lock,
+    #[serde(alias="Mod1")]
+    Alt,
+    Mod2,
+    Mod3,
+    Mod4,
+    Mod5,
+}
+
 #[derive(Debug, Clone, Deserialize, PartialEq)]
 #[serde(deny_unknown_fields)]
 struct Outline {
@@ -510,22 +530,27 @@ fn create_action<H: logging::Handler>(
         Action(Action),
         Text(String),
         Keysym(String),
+        Modifier(Modifier),
     };
     
     let submission = match (
         &symbol_meta.action,
         &symbol_meta.keysym,
-        &symbol_meta.text
+        &symbol_meta.text,
+        &symbol_meta.modifier,
     ) {
-        (Some(action), None, None) => SubmitData::Action(action.clone()),
-        (None, Some(keysym), None) => SubmitData::Keysym(keysym.clone()),
-        (None, None, Some(text)) => SubmitData::Text(text.clone()),
-        (None, None, None) => SubmitData::Text(name.into()),
+        (Some(action), None, None, None) => SubmitData::Action(action.clone()),
+        (None, Some(keysym), None, None) => SubmitData::Keysym(keysym.clone()),
+        (None, None, Some(text), None) => SubmitData::Text(text.clone()),
+        (None, None, None, Some(modifier)) => {
+            SubmitData::Modifier(modifier.clone())
+        },
+        (None, None, None, None) => SubmitData::Text(name.into()),
         _ => {
             warning_handler.handle(
                 logging::Level::Warning,
                 &format!(
-                    "Button {} has more than one of (action, keysym, text)",
+                    "Button {} has more than one of (action, keysym, text, modifier)",
                     name,
                 ),
             );
@@ -614,6 +639,26 @@ fn create_action<H: logging::Handler>(
                 })
             }).collect(),
         },
+        SubmitData::Modifier(modifier) => match modifier {
+            Modifier::Control => action::Action::ApplyModifier(
+                action::Modifier::Control,
+            ),
+            Modifier::Alt => action::Action::ApplyModifier(
+                action::Modifier::Alt,
+            ),
+            unsupported_modifier => {
+                warning_handler.handle(
+                    logging::Level::Bug,
+                    &format!(
+                        "Modifier {:?} unsupported", unsupported_modifier,
+                    ),
+                );
+                action::Action::Submit {
+                    text: None,
+                    keys: Vec::new(),
+                }
+            },
+        },
     }
 }
 
@@ -711,6 +756,7 @@ mod tests {
                         keysym: None,
                         action: None,
                         text: None,
+                        modifier: None,
                         label: Some("test".into()),
                         outline: None,
                     }
@@ -852,6 +898,7 @@ mod tests {
                         keysym: None,
                         text: None,
                         action: None,
+                        modifier: None,
                         label: Some("test".into()),
                         outline: None,
                     }