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/vkeyboard.rs

@@ -1,6 +1,6 @@
 /*! Managing the events belonging to virtual-keyboard interface. */
 
-use ::keyboard::{ KeyCode, PressType };
+use ::keyboard::{ KeyCode, Modifiers, PressType };
 use ::layout::c::LevelKeyboard;
 use ::submission::Timestamp;
 
@@ -26,6 +26,11 @@ pub mod c {
             virtual_keyboard: ZwpVirtualKeyboardV1,
             keyboard: LevelKeyboard,
         );
+        
+        pub fn eek_virtual_keyboard_set_modifiers(
+            virtual_keyboard: ZwpVirtualKeyboardV1,
+            modifiers: u32,
+        );
     }
 }
 
@@ -33,7 +38,7 @@ pub mod c {
 pub struct VirtualKeyboard(pub c::ZwpVirtualKeyboardV1);
 
 impl VirtualKeyboard {
-    // TODO: split out keyboard state management
+    // TODO: error out if keymap not set
     pub fn switch(
         &self,
         keycodes: &Vec<KeyCode>,
@@ -68,12 +73,16 @@ impl VirtualKeyboard {
         }
     }
     
+    pub fn set_modifiers_state(&self, modifiers: Modifiers) {
+        let modifiers = modifiers.bits() as u32;
+        unsafe {
+            c::eek_virtual_keyboard_set_modifiers(self.0, modifiers);
+        }
+    }
+    
     pub fn update_keymap(&self, keyboard: LevelKeyboard) {
         unsafe {
-            c::eek_virtual_keyboard_update_keymap(
-                self.0,
-                keyboard,
-            );
+            c::eek_virtual_keyboard_update_keymap(self.0, keyboard);
         }
     }
 }