Add option to convert Qt4-based VS project

There is no automated procedure to migrate a Qt4-based Visual Studio project to Qt5. It can be done manually, by updating all relevant configurations. But this is somewhat tedious / error-prone.

When opening a Qt project in Visual Studio, the Qt VS tools should:

1. Check if the project uses Qt4
2. If it does, show a warning message, prompting the user to confirm migration to Qt5
3. If the user confirms, modify all relevant build process properties (e.g. additional include dirs, .lib names, etc.)
4. Open the Qt version selection dialog, to allow the user to choose the Qt5 version that will be used henceforth

Note that this migration procedure would only affect the build process; required changes to the source code (cf. Transition from Qt 4.x to Qt5) would still need to be done by hand.