MinGW mtests on Windows with QtCreator

[jeetee]

Resolves: Enable mtest for MinGW on Windows

• Ensures dependent files to run a test are installed by that test
• Removes dependency on having installed mscore first (mtest_build_and_install target can be run on a clean clone)
• Removes duplicate commands only due to a different mtest project dir for MSVC
• Introduces new build target mtest_build_and_install for easier configuration from within an IDE (such as QtCreator)

See https://musescore.org/en/node/307131 for a writeup of the QtCreator configuration (starting from "Enabling mtests")

Hopefully will turn out to apply quite cleanly to master as well (untested)

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [N/A] I created the test (mtest, vtest, script test) to verify the changes I made
• I ran the related test(s) on my local machine successfully

[vpereverzev]

@jeetee looking for rebase

[jeetee]

@vpereverzev done