Add keyboard modifiers to input event

Making applications track modifier keys results in unnecessary work for
consumers, it's error prone, and it turns out to have unavoidable bugs.
For example, alt-tabbing with x11 results in the alt modifier state
getting stuck.

To resolve these problems, this patch adds a Mods value to the keyboard
input event.

Based on this patch: d287fa96e3

src/api/wayland/keyboard.rs

@@ -3,6 +3,8 @@ use std::sync::{Arc, Mutex};
 
 use {VirtualKeyCode, ElementState, WindowEvent as Event};
 
+use events::ModifiersState;
+
 use super::wayland_kbd;
 use wayland_client::EventQueueHandle;
 use wayland_client::protocol::wl_keyboard;
@@ -35,7 +37,8 @@ impl wayland_kbd::Handler for KbdHandler {
             };
             let vkcode = key_to_vkey(rawkey, keysym);
             let mut guard = eviter.lock().unwrap();
-            guard.push_back(Event::KeyboardInput(state, rawkey as u8, vkcode));
+	    // TODO implement ModifiersState
+            guard.push_back(Event::KeyboardInput(state, rawkey as u8, vkcode, ModifiersState::default()));
             // send char event only on key press, not release
             if let ElementState::Released = state { return }
             if let Some(txt) = utf8 {