Check for extensions versions consistency

When users have different versions of extensions, we should warn them or give them an error. Otherwise, some features might not work, or runtime problems might occur.

For example, let's say `qt-qml` is 1.6 and `qt-cpp` is 1.4. If some features are added to `qt-qml` with 1.6 which requires features from `qt-cpp` 1.6. This can cause `qt-qml` 1.6 feature not to work completely. 

Possible solution: 

qt-core can check the other extensions' versions and show an error if there is inconsistency.