Ensure MinGW kits work in Qt Creator

Once MinGW support is added in QUL core, including json kit files, we must verify that the integration in Qt Creator is working.

Acceptance criteria:

• MinGW kits are automatically recognized by the MCU plugin 
• MinGW kits can be created from the MCU settings page, just like other kits
• MinGW kits are only visible and usable on Windows
• MinGW kits can successfully be used to build QUL applications from Qt Creator.