Port Wayland clients to new xkbcommon API

A little different from Daniels initial patch.  We look up the common
modifiers at xkb init time and convert the xkb serialized modifier mask
to our own modifier bitmask.
diff --git a/clients/window.h b/clients/window.h
index ac52935..8274e2a 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -360,6 +360,10 @@
 void
 input_get_position(struct input *input, int32_t *x, int32_t *y);
 
+#define MOD_SHIFT_MASK		0x01
+#define MOD_ALT_MASK		0x02
+#define MOD_CONTROL_MASK	0x04
+
 uint32_t
 input_get_modifiers(struct input *input);