- Patch #13738 by TDobes: theme system fixes:

* Fix a bug which would cause the "configure" link for styles to be broken.
* Fix a bug with using drupal_get_filename for theme engines. Although this is not called anywhere in core, we should still fix it for contrib. (i.e. themes that may want to manually invoke a theme engine to create a hybrid theme)
* Correct an inaccurate comment in theme.inc
* Populate the default primary links with an "edit primary links" link for consistency with the secondary links
* remove some unnecessary variables in the theme administration which had misleading and confusing names
* replace time-consuming foreach when rendering theme admin page with a more efficient array_key_exists
* usability: rather than completely removing the search box checkbox when search.module is disabled, simply disable it. (UI elements shouldn't appear/disappear.)