Now you can click on URLs

If true, load the extensions on startup. Otherwise, don't load
them automatically.

This flag will indicate that we can't load the extensions module,
thus preventing many unnecessary (failing) loads on ViewMenu::exec()

Clean Extension dialogs and widgets without using a cond_timedwait but
a simple cond_wait instead.

The QPixmapCache default size is 10240kB on desktops, this leaves room
for 640 pictures 64x64 in rgba

Improves scrolling with a lot of items.

Also use art url from the first children with art for nodes without art

TODO: cache the full rendering (text + art), QPixmap is a QPaintDevice
subclass

Only respond to input item change if in icon view.
Update location bar according to current view.

And add a debug msg when sorting happens.

When RTSP is shut down, the server destroys sessions (with no locking)
while clients are still able to concurrently access them, potentially
leading to a crash. Make sure we unregister the RTSP URL before
cleaning up (then indeed no locking is needed).

This is ugly, but I don't know how to do it the correct way.
The UI is not top-quality yet, but it works. Feel free to improve.

When an ES is removed, it is possible to set up a track that won't be
cleaned up and will remain dangling, causing a crash later. Make sure
we unregister the RTSP URL before cleaning up.