clutter/x11: Consider remapped keys when guessing the keycode from the keysym

Since e3e933c4 a keyval can be temporarily remapped to an unused keycode. Due to some limitations in XTestFakeKeyEvent, the remapping has to be done in the first xkb group/layout. In case there are two or more keyboard layouts enabled and the selected keyboard layout is not the first, clutter_keymap_x11_keycode_for_keyval will fail to retrieve the correct keycode for a remapped keyval. Let's use the reserved_keycodes map in order to retrieve the correct keycode if needed.

Fixes: #443 (closed)