Merge St.TextureCache and Shell.TextureCache

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