src/core/monitor-xrandr.c: Rewrite under/overscan transition

Reimplement transition using …_xrandr_apply_configuration:
Keeps the XRandR configuration logic all in one place instead of
duplicating it.
Merge pull request #21 from fledermaus/eos-shell-underscan-overscan-rewrite

src/core/monitor-xrandr.c: Rewrite under/overscan transition