Expire WP_Theme cache on updates and theme edits

[nacin]

When saving a file in wp-admin/theme-editor.php, we should force a cache delete.

Additionally, cache deletes should occur when an update occurs. Otherwise we may be reading old Version numbers from disk.

When deleting the cache, we will need to refresh the headers that exist in the current object instance. A simple $this->construct() can re-initialize everything. This will enable us to call $theme->cache_delete() at any time to force fresh information.