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/wayland.c

@@ -20,6 +20,12 @@ void eek_virtual_keyboard_update_keymap(struct zwp_virtual_keyboard_v1 *zwp_virt
         keyboard->keymap_fd, keyboard->keymap_len);
 }
 
+void
+eek_virtual_keyboard_set_modifiers(struct zwp_virtual_keyboard_v1 *zwp_virtual_keyboard_v1, uint32_t mods_depressed) {
+    zwp_virtual_keyboard_v1_modifiers(zwp_virtual_keyboard_v1,
+                                      mods_depressed, 0, 0, 0);
+}
+
 int squeek_output_add_listener(struct wl_output *wl_output,
                                 const struct wl_output_listener *listener, void *data) {
     return wl_output_add_listener(wl_output, listener, data);