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/st/st-types.h

@@ -42,6 +42,13 @@ typedef enum {
   ST_ALIGN_END
 } StAlign;
 
+typedef enum {
+  ST_ICON_SYMBOLIC,
+  ST_ICON_FULLCOLOR,
+  ST_ICON_APPLICATION,
+  ST_ICON_DOCUMENT
+} StIconType;
+
 G_END_DECLS
 
 #endif /* __ST_TYPES_H__ */