Improve moving through arrow keys

[massimo ceraolo]

Moving diagram parts through arrow keys is often convenient with respect to using the mouse because it gives more control.
Using several keystrokes in a row would in principle allow fast and precise positioning of diagram parts.

Unfortunately, moving diagram parts by keyboard has never been very fast with OM and is still rather slow, see #5620. Especially moving through several keystrokes in a row is made ineffective because this, probably unavoidable, slowness, is multiplied by the number of keystrokes.

A very good improvement on this could be brought by avoiding interacting with OMC on any of these keystrokes.

I mean, when OMEdit intercepts arrow keystrokes it could wait some time, say 200-300ms, to see if others follow. Only after the end of an arrow keystroke sequence, it could interact with OMC, so only once, asking for the entire movement corresponding to the sequence. In this way, the waiting time would be nearly the same as a single keystroke movement, i.e. movement would be much faster than today.