The -T parameter to CMake may now be specified through cmake-gui via a
new text field in the first-time configure wizard (below the generator
chooser).
The generator factories specify whether or not they support toolsets.
This information is propagated to the Qt code and used to determine if
the selected generator should also display the optional Toolset widgets.
Pass the name of the requested generator to the generator factory,
which is now responsible to check if it can create a matching
generator for the name. This allows us to add more logic to the
factory in a next step, so that not every possible generator needs
to get registered explicit in cmake::AddDefaultGenerators().