[Windows] Add MinGW generator to global CMake settings page

Summary:
In cmakeutils.cpp:
-Added "MinGW Makefiles" to the stringlist generatorNames.
In cmakebuilder.cpp
-Added "MinGW Makefiles" to stringlist required for addBuilder(Makefile)

Test Plan:
-Pruned a small helloworld project, selected MinGW Makefiles as the default generator,
configures fine, mingw32-make succeeds and executable launches as expected.

Please report if you encounter any errors.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6638