QML debugging fro QML apps is disabled (by default)

When using the QtQuick Application wizards (or when oenening a QtQuick example), the project option to debug QML is now dsiabled, and C++-debugging is enabled.

That usability regression was introduced by https://codereview.qt-project.org/#change,78949
Effectively, this means that Qt Creator 3.1 beta has QML debugging disabled. Only who knows the three magic clicks to enable ti for a project/kit will be able to debug QML

The background of https://codereview.qt-project.org/#change,78949 was that users who open a C++ project that has some QtQuick somewhere would get the mixed c++/qml debug mode, which is inconvenient for pure C++ debugging. Qt Creator for example has a hierarchy of SUBDIR projects. And some of the leaf LIB projects us QML, such as WelcomeScreen, Designer and Profiler.

In order to determine which debug mode to pre-select, Qt Creator seems to scan the whole projects tree to detect any usage of QtQuick.

Agreed solution:

• We make sure that runConfiguration()->target()->project()->projectLanguages() only considers the current .pro file and looks for "QT += quick" or qml or declarative
• If the root .pro file of the open project does not contain any of those, we pre-select C++ debugging, otherwise, we chose mixed debugging
• We revert https://codereview.qt-project.org/#change,78949