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/qt4projectmanager/qt-s60/s60manager.cpp

@@ -131,9 +131,7 @@ S60Manager::S60Manager(QObject *parent)
     addAutoReleasedObject(new S60CreatePackageStepFactory);
     addAutoReleasedObject(new S60DeployStepFactory);
 
-    addAutoReleasedObject(new RunControlFactory<S60DeviceDebugRunControl, S60DeviceRunConfiguration>
-                          (QLatin1String(Debugger::Constants::DEBUGMODE),
-                           tr("Debug on Device"), parent));
+    addAutoReleasedObject(new S60DeviceDebugRunControlFactory);
     addAutoReleasedObject(new Qt4SymbianTargetFactory);
 
     updateQtVersions();