Add support of MinGW on AppVeyor

[misery]

@chcg Could you look into this, too?

• Do you really need "_xp"? Qt5 do not support Windows XP anymore. Also it is not supported by AusweisApp2.
• "-- /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"" cannot be used with MinGW. I don't see any difference with MSVC if I remove this.
• Is toolset necessary? Visual Studio 2015 and 2017 will use their default toolset if not defined by cmake.

[chcg]

@misery Looks fine.

• If XP is no target and not supported any longer by Qt5 removal is fine, just copy&paste from some different projects
• toolset would just be necessary in the XP use case, because the default toolset would be just e.g. v140 and not v140_xp. So removal is also fine.
• the logger should add the errors/warnings to the message tab, see e.g. https://ci.appveyor.com/project/donho/notepad-plus-plus/build/1.0.1682/job/61ld8xdjuaonad2a/messages, but seems to not work as expected anyhow so removal should not hurt

[misery]

Thanks!
Ok, now I know what the logger does. It works: https://ci.appveyor.com/project/GovernikusAusweisApp2/ausweisapp2/build/1.14.0-6/job/q0a43ytp6b1jfkc4/messages
Can be fixed by: nodejs/http-parser#296

The other builds do not display warnings because we enable /WX and -Werror here and fixes warnings.

Would be nice to add the logger to MinGW build, too.

[chcg]

See https://www.appveyor.com/docs/build-worker-api/#add-compilation-message. To my current knowledge just the MSBuildLogger is available. I'm not sure how the mingw build behaves with the logger, but in case of problems it should be possible to just add it as optional parameter set within the build matrix