Make revert direction functionality available from menu.

This patch makes the ::revertDirection method available from the menu (with undo-functionality). Since my first patch was to visualize the direction (by giving start/end handles different colors), this is the next step as it allows to revert the direction of lines and arcs.

The icon given in rs_actionmodifyrevertdirection.cpp is missing, I am totally hopeless when it comes to graphics, maybe someone else can create an icon?

My next step will be to implement ::revertDirection for polylines and splines.