Debugger: Rework step{In,Out,Over} handling

Main menu action pass operation to current engine, everything else
is handled there.

Combine execute{Step,Next} and execute{Step,Next}I functions.
Implementation were mostly similar, in some cases unneeded
(the instruction-wise version e.g. for Python)

Drop GDB-isms 'step', 'next' in favor of 'step in' and 'step over'.

Change-Id: I232232bc7a67d9d297a74f1c81dc43be96787d34
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/disassembleragent.cpp

@@ -400,6 +400,7 @@ void DisassemblerAgent::updateBreakpointMarker(const Breakpoint &bp)
 
     auto marker = new DisassemblerBreakpointMarker(bp, lineNumber);
     d->breakpointMarks.append(marker);
+    QTC_ASSERT(d->document, return);
     d->document->addMark(marker);
 }