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/keyboard.rs

@@ -23,6 +23,24 @@ pub enum PressType {
 
 pub type KeyCode = u32;
 
+bitflags!{
+    /// Map to `virtual_keyboard.modifiers` modifiers values
+    /// From https://www.x.org/releases/current/doc/kbproto/xkbproto.html#Keyboard_State
+    pub struct Modifiers: u8 {
+        const SHIFT = 0x1;
+        const LOCK = 0x2;
+        const CONTROL = 0x4;
+        /// Alt
+        const MOD1 = 0x8;
+        const MOD2 = 0x10;
+        const MOD3 = 0x20;
+        /// Meta
+        const MOD4 = 0x40;
+        /// AltGr
+        const MOD5 = 0x80;
+    }
+}
+
 /// When the submitted actions of keys need to be tracked,
 /// they need a stable, comparable ID
 #[derive(PartialEq)]