* Fixed broken deletion of index file in clear_thumbdir() and made it safer.

* Fixed broken deletion of index file in clear_thumbdir() and made it safer.
* Thumbnail manager now refuses to remove folders containing files that aren't ours.
* Added 'Invert Selection' and 'Select Broken' buttons to thumbnail manager.
* Improved button layout, styling and logic in thumbnail manager.
* Reset 'force' flag after initial load_view() to avoid nasty surprises.
* Added 'Force Rebuild' option to context menu.
* Minor tweaks: small optimizations, code simplification, visual improvements, debugging, whitespace.