Recursively add directories when adding files.
AddTaskWizard: reuse Extension object.
Extensions: add contains() method.
Ignore file extensions not known as media files in recursion.

When recursively adding files, exclude every file whose extension
is not in the multimedia extension list (union of video and audio).
AddTaskWizard: always show the "Files" page.
ConvertList: call addTasks() only if wizard returns Accepted.
AddTaskWizard: remove dependency to QUrl.
Filter out unknown extensions in drag-and-dropped files.
Drag and drop: don't show AddTaskWizard if no file is accepted.
Fix drag-and-drop directories.
Don't filter dropped files.

If the user insists on dropping a file whose extensions is not
recognized, don't reject that file. Only filter file by extension when
recursively adding a directory.
AddTaskWizard: extract addFiles() function.
AddTaskWizard: show a progress dialog if adding files takes more than 100ms.