Move operations as late/early as possible

Right-click menu in scheduler GUI: "Move as late as possible", "Move as early as possible".

Should check for maxint and possibly not enable in case the slack is zero.

One may consider implementation this on the Schedule class as well. Would give an easy way to obtain an ALAP schedule from an ASAP schedule, by calling move_as_late_as_possible(graph_id) (better name?) in opposite precedence order.