Juk should be able to exclude folders [patch]

[Tom Li]

If we add some folders to Juk's watching list, Juk will add everything
in the folder. But sometimes, we don't want Juk to add some folders.
There isn't a way to exclude folders.

It is bugging me all the time. I can't stand it so I starting patching
it with a little C++ and Qt knowledge.

Here is my patch. If my patch can merge into mainline, our users
should be happier :). I don't have much C++ experience, there are some
hacks in my code, but that's the best what I can do. So please help me
to improve it.

I opened a bug (https://bugs.kde.org/show_bug.cgi?id=319106), you can
see my patch at there.