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/runDialog.js

@@ -7,9 +7,9 @@ const GLib = imports.gi.GLib;
 const Lang = imports.lang;
 const Mainloop = imports.mainloop;
 const Meta = imports.gi.Meta;
+const St = imports.gi.St;
 const Shell = imports.gi.Shell;
 const Signals = imports.signals;
-const St = imports.gi.St;
 const Gettext = imports.gettext.domain('gnome-shell');
 const _ = Gettext.gettext;