Make it possible for `step --over` to stop on thread switch

[Vladimír Štill]

Currently step --over continues until the execution of the current statement and all calls in it terminates or a breaking action, such as fault is encountered. This can be rather unintuitive if thread switching occurs -- the other thread continues running freely until it either switches back to the original and statement ends or there is a fault. I suggest making step --over stop also on thread switching and adding another option to step which also steps over other threads. Note that there is currently no way of executing a statement and stopping at thread boundary.