This is a pretty straight forward change to remove the global front
variable and instead use a couple of accessor functions.
This hopefully will make it easier to keep track and debug when
the front screen changes.
Although it likely doesn't matter /that/ much, the x11 xlib manual
does say to use it rather than expecting the struct to stay the same
size.
Yeah yeah it's 2022 and we really do build things from source,
ICCCM is likely not going to change that much nowdays, etc, etc.
