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/action.rs

@@ -10,8 +10,11 @@ pub struct KeySym(pub String);
 type View = String;
 
 /// Use to send modified keypresses
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum Modifier {
+    /// Control and Alt are the only modifiers
+    /// which doesn't interfere with levels,
+    /// so it's simple to implement as levels are deprecated in squeekboard.
     Control,
     Alt,
 }
@@ -27,8 +30,8 @@ pub enum Action {
         /// When unlocked by pressing it or emitting a key
         unlock: View,
     },
-    /// Set this modifier TODO: release?
-    SetModifier(Modifier),
+    /// Hold this modifier for as long as the button is pressed
+    ApplyModifier(Modifier),
     /// Submit some text
     Submit {
         /// Text to submit with input-method.