Support spanning Tilda across all monitors

[Mr-Tao]

Fixes #299.

Summary

• add an All monitors entry to the monitor chooser;
• calculate percentage-based geometry and centering from the bounding rectangle of all monitor workareas when selected;
• use the same rectangle for the pull-down animation, and recalculate size, position, and animation coordinates after monitor/workarea changes.

Existing configurations retain the per-monitor behavior because span_monitors defaults to false.

Testing

• make
• make check
• XFCE/X11 live test with eDP-1 at 1920x1080+0+0 and DP-1-2 at 2560x1440+1920+0:
  • 100% width / 50% height: 4480x719+0+33
  • after disabling eDP-1: 2560x703+0+33
  • after restoring eDP-1: 4480x719+0+33