cmake-gui: Fix cmState initialization when changing generators (#15959)

Refactoring in commit v3.3.0-rc1~29^2~1 (cmState: Host some state from
the cmGlobalGenerator, 2015-05-24) moved storage of some generator
traits over to cmState.  However, it accidentally removed initialization
of the values from the cmGlobalGenerator constructor.  This is needed
because generator subclasses update the settings in their constructors.
Since a single cmState instance is shared across multiple build trees by
cmake-gui, initializing the values in its constructor is not enough.
Fix this by restoring the needed initializations to the
cmGlobalGenerator constructor.