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

js/ui/appDisplay.js

@@ -694,7 +694,7 @@ AppIconMenu.prototype = {
         this._windowContainer.show();
 
         let iconsDiffer = false;
-        let texCache = Shell.TextureCache.get_default();
+        let texCache = St.TextureCache.get_default();
         if (windows.length > 0) {
             let firstIcon = windows[0].mini_icon;
             for (let i = 1; i < windows.length; i++) {