Remove "window type" window rule

While the window rules allow changing the window type, our code is written in a way that assumes that it's static. Fixing that code to handle window type would bring undesired complexity, for example the show desktop would need to monitor for window type changes and clean up the Window::hiddenByShowDesktop() property when needed.

Less error prone option is to make the window type static.

Keep above, keep below and accepts focus rules partially cover what the window type rule can be used for. I believe the only missing piece is to provide a window rule to overwrite the stack layer. With that, it should be safe-ish to drop the window type rule.