Debugger: Display JS/QML stack frame when a Quick 2 application crashes/stops

When a Quick2 application crashes, it is hard to find out from which QML file the crash originated. In the default case, the debugger just shows a bit of V4 stack and then a garbled trace. It would be good if the QML file could be shown.

• A checkbox to set QV4_FORCE_INTERPRETER could be added (somewhere next to QML debugging in the run configuration . This would result in proper stack traces

• The QML file/line number could be extracted from a variable of type QV4::ExecutionContext which can be found in the locals, or deduced from the stack pointer with some offset or by retrieving a register value. A free function taking that pointer and returning an allocated string of the form "file:line" could be added to QtQml (see ExecutionEngine::stackTrace(int frameLimit)). The debugger could then call that to open an editor at that point.