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.

'one-instance' happens to work on Win32 though there are several issues:
- a WM_QUIT is sent to the helper thread when any instance terminates
 (the master or a secondary instance). 'one-instance' should then stop
 working as soon as the first secondary instance terminates.
- But, sending WM_QUIT via SendMessage directly calls the window
procedure callback. And this callback here doesn't process the message
at all. Therefore, it is a no-op and the thread is actually never stopped.

This patch does the following :
- move the WM_QUIT message to ensure that only the master (first) instance
 stops the helper thread.
- process the WM_QUIT message in the window procedure callback, and call
for clean termination of the thread.

Note that PostQuitMessage cannot be directly called as there are two
distincts threads here.