context menu: move "Remove from library" to "Manage..." sub menu
context menu: use string-based media type detection instead of VIDEO_DB_CONTENT_FOO
context menu: add Manage... sub-menu to movie sets
context-menu: add "Manage..." sub-menu to tags
CGUIDialogVideoInfo: add NULL-pointer and other sanity checks
context menu: move "Scan for new content" to the very bottom for tvshows (same as for video sources)
context menu: move "Play" to the top (for folders) as it should always be the first option