Convert Weston modifier #defines to an enum

To avoid any possible collision between the disparate XKB and Weston
modifier namespaces.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/src/shell.c b/src/shell.c
index 4d11ba4..fcac599 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -242,7 +242,7 @@
 center_on_output(struct weston_surface *surface,
 		 struct weston_output *output);
 
-static uint32_t
+static enum weston_keyboard_modifier
 get_modifier(char *modifier)
 {
 	if (!modifier)