Fixes and improvements when using MinGW

[FMeinicke]

I wanted to integrate your plugin into our application and came across some issues when using MinGW to build your project:

• The CMake check for the VS compiler version also rejected the MinGW compiler Being on a Windows OS does not automatically mean that Visual Studio compiler is used. If the compiler is MinGW's gcc for example the check for the Visual Studio compiler version wouldn't have allowed this. This is now fixed by checking for MSVC instead of WIN32 before checking the actual version of the compiler

• Because the resulting DLLs were named incorrectly the QML Engine wouldn't recognize them When compiling the plugin with MinGW on Windows the output name of the resulting libraries (DLLs) always starts with "lib". However, in order for the QML engine to find the plugin DLLs they have to be named exactly as specified in the qmldir file, that is without any prefix just 'StylePlugin.dll' respectively 'AqtTestUtilsPlugin.dll'

[ala-ableton]

@FMeinicke thank you very much for the PR! Before we can accept your contribution, we need you to sign a Contributor License Agreement (CLA). Please head to http://ableton.github.io/cla/ to do so. Thanks!

[FMeinicke]

All done!

[gck-ableton]

Nice!

@gck-ableton would you like to have a look as well?

I did. The changes look good! Thanks for the work!