kcachegrind: port context menu of call graph to QMenu (part 1)

This convertion to QMenu/QAction also gets rid of some sanity checking of configuration parameters, which (1) was misplaced here and (2) is not needed.

Because of the huge context menu of the call graph, this is done in a few steps, ie. further parts coming...

I admit that the resulting code is probably easier to understand (no fiddling with menu item IDs), and no large switch statement any more.

But the introduction of a lot of new QActions and new slots to handle triggered actions makes the code not really smaller.