Automatically "autoconf" screen-sized new windows as "allscreened"

This enables automagic mplayer -fs borderless fullscreened playback windows
with minor additions to the code.  Previously one had to focus the playback
window and mod1-triple-k it to lose the borders, now it "just works".
Merge pull request #1 from vcaputo/auto_allscreen

Automatically "autoconf" screen-sized new windows as "allscreened"