Clean up code to find themes

Simplify code to find the right theme to load and loading it by moving
all the loading code into a load_theme() helper function, and making
meta_load_theme() use that as it searches through the directories.

Look for old-version themes even when loading relative to the working
in debug mode.

Don't unnecessarily duplicate and then free info->theme_file and
info->theme_dir.

http://bugzilla.gnome.org/show_bug.cgi?id=592503