i3companion/randr: Cache screen resources for a few seconds
i3companion/randr: Cache screen resources for a few seconds

randr.GetScreenResources() takes about 20ms to complete on my Thinkpad,
which is quite noticeable, especially when repeatedly tapping the
brightness keys, or holding them down.

This will cache the result of GetScreenResources for up to five seconds,
making things a whole lot smoother and responsive.

DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk