Merge St.TextureCache and Shell.TextureCache

Brute force merge these two by essentially replacing St.TextureCache
with a (renamed) Shell.TextureCache.

One function was added for convenience, namely "st_texture_cache_load_file_simple".
St.TextureCache had a function to load a texture from a filename, and it
returned NULL on error but only half the callers actually checked this.  This
function is better.

https://bugzilla.gnome.org/show_bug.cgi?id=607500

src/shell-window-tracker.c

@@ -16,7 +16,7 @@
 #include "shell-window-tracker.h"
 #include "shell-app-system.h"
 #include "shell-app-private.h"
-#include "shell-texture-cache.h"
+#include "st/st-texture-cache.h"
 #include "shell-global.h"
 #include "shell-marshal.h"
 
@@ -847,8 +847,8 @@ shell_startup_sequence_create_icon (ShellStartupSequence *sequence, guint size)
     }
 
   themed = g_themed_icon_new (icon_name);
-  texture = shell_texture_cache_load_gicon (shell_texture_cache_get_default (),
-                                            themed, size);
+  texture = st_texture_cache_load_gicon (st_texture_cache_get_default (),
+                                         themed, size);
   g_object_unref (G_OBJECT (themed));
   return texture;
 }