Better preventing of pop up windows

Current approach of handling this is to raise slimlock root window every second from a dedicated thread. For some reason it didn't actually work for me. I never figured out why, but I suspect it has something to do with notorious xlib threading support.

My change is twofold. First, it adds handling of visibility events. Thus when compositing is not used, slimlock will almost instantly raise itself whenever some window pops up on top of it. Second, since this approach doesn't work with compositing, the old logic of raising root window periodically is somewhat preserved. But I moved it into event loop (so no more separate thread). Additionally, I made the raise interval configurable.