Add multi-monitor awareness

 * Previously it was possible to move the tilda window
   to the right monitor and resize it appropriately by
   using the x- and y-position spinners. But tilda did
   not have any particular awareness of the different
   monitors.
 * This patch addes a combo box to choose the monitor
   number and the necessary logic to constrain the
   tilda window to the current monitor. It is no longer
   possible to span tilda over two monitors, but that
   ability was probably not usefull in the first place
   anyway.
 * Changing the monitor will resize tilda according to the
   current percentage of height and width based on the new
   monitor size, and position the window according to the
   offset of the monitor.
 * Also add a lot of documentation for different functions.
 * Fixes #3