debugger: create a disassembler view as main editor

"Stepping instruction wise" and "display disassembler" mode is now toggled by a
single action. This also allows to re-use F10 and F11 as keyboard shortcut.

Missing: caching of disassembler output, removal of old disassembler view.

src/plugins/debugger/debuggeractions.h

@@ -77,6 +77,7 @@ enum DebuggerActionCode
     AutoQuit,
     LockView,
     LogTimeStamps,
+    StepByInstruction,
 
     RecheckDebuggingHelpers,
     UseDebuggingHelpers,