Move StIconType to st-types.h from st-texture-cache.h

StIconType will be used by a new StIcon class, so move it to the
header file of common enumerations. Including st-types.h which had
the St single-include check revealed that st-texture-cache.h didn't
have that check and several places were including that directly.
Fix that up.

https://bugzilla.gnome.org/show_bug.cgi?id=633865

src/shell-app-system.c

@@ -13,8 +13,8 @@
 
 #include "shell-app-private.h"
 #include "shell-global.h"
-#include "st/st-texture-cache.h"
 #include "display.h"
+#include "st.h"
 
 #define GMENU_I_KNOW_THIS_IS_UNSTABLE
 #include <gmenu-tree.h>