#1935, 1988 - Make FFmpeg easier to install, add allow_movie_uploads option

Make FFmpeg easier to install:
- movie::allow_uploads (new) - return true if allow_movie_uploads is "always" or "default" and FFmpeg found, false otherwise
- legal_file, Form_Uploadify - use movie::allow_uploads instead of movie::find_ffmpeg
- system::find_binary - add Gallery's bin subdirectory to search
- system::find_binary - auto-fix permissions if found in Gallery directory

Add allow_movie_uploads option:
- admin_movies, admin_movies.html (new) - new Movies admin screen
- movie::find_ffmpeg_version (new) - return version number and date of FFmpeg
- graphics::mark_dirty - added $type argument to rebuild only one type (e.g. "movie")
- form_uploadify.html - change text if movie uploads are disabled
- gallery_event::admin_menu - added Movies link to Settings
- gallery_installer, module.info, install.sql - add allow_movie_uploads variable (NOT FINISHED)