Port lxqt-panel to KWindowSystem

Keyboard indicator and System tray still need Xlib/xcb, and the urgency hint behavior is not working, as [KWindowSystem does not support it properly yet](http://lists.kde.org/?l=kde-core-devel&m=140991729804249&w=2).

I'm dropping Qt4 support here, as discussed in the ML. This is not targeted at 0.8 release anyway.

We shall decide how we are going to approach the system tray issue, as KWindowSystem will not support it (as seen [here](http://blog.martin-graesslin.com/blog/2014/03/system-tray-in-plasma-next/) and [here](http://blog.martin-graesslin.com/blog/2014/06/where-are-my-systray-icons/)).