[xmonad] [patch] Added focusMaster to BoringWindows

Hey! This is a little patch to add 'focusMaster' to BoringWindows. If used instead of the normal StackSet.focusMaster it will focus the master window unless it's 'boring' in which case the next non-boring window will be focused. While working on the patch, I noticed that the core provides focusUp' and focusDown' that work on a Stack rather than an entire StackSet, but there is no focusMaster' which would do the same thing. As I needed something like that for the modification, I added it to the core. The patch for this is also attached. But I can also do it without touching the core, if something speaks against that. Regards! Jan