Debugger: Add toolchain combos to start dialogs, refactor detection.

Add combo box listing toolchains with debuggers to start external,
attach and core.
Another attempt at streamlining engine detection:
Split in detection functions that first collect a list of
available engines by preference, then remove disabled and
wrongly configured engines and use the remaining best.
matching. checkconfiguration is now the central place where
engine detection and config check takes place.

Rubber-stamped-by: hjk

src/plugins/debugger/qml/qmlcppengine.h

@@ -48,7 +48,9 @@ class DEBUGGER_EXPORT QmlCppEngine : public DebuggerEngine
     Q_OBJECT
 
 public:
-    explicit QmlCppEngine(const DebuggerStartParameters &sp);
+    explicit QmlCppEngine(const DebuggerStartParameters &sp,
+                          DebuggerEngineType slaveEngineType,
+                          QString *errorMessage);
     ~QmlCppEngine();
 
     bool setToolTipExpression(const QPoint &mousePos,