Add a configurable way to raise slimlock window periodically.

This change supplements a previous attempt to handle popping windows
better. I realized that approach based on mere handling of visibility
events doesn't work when compositing is enabled. Thus it's still
needed to raise slimlock window from time to time. So I added
`raise_interval' configuration option which specifies how often the
window must be raised (default value is every 1000ms). It's possible
to disable raising completely by setting `raise_interval' to
zero. Everything is implemented in a single thread to avoid notorious
Xlib multi-threading issues.