Use modern CMake targets for SDL2

[DomClark]

This PR changes FindSDL2.cmake to provide an imported target SDL2::SDL2 to link to, rather than SDL2_LIBRARY and SDL2_INCLUDE_DIR variables to manually target. This is the same as provided by SDL2's own CMake configuration script, which means that that script can be used in place of ours if available. Indeed, that is the motivation - our script only imports a single version of SDL2, which means that MSVC debug builds of LMMS still use the release version of SDL2. With SDL2's own multiple imported configurations, CMake can select the correct version of the library to use with a particular build configuration.

This is a step towards solving #5683 - it fixes the problem for Debug builds, and simplifies the situation for RelWithDebInfo builds, which should make a full fix easier in the future.

Other changes made:

• SDL2's own configuration script is now used if available, falling back to ours if no SDL2::SDL2 target is defined (either because the script is missing, or because it's a custom one that still uses old CMake variables).
• Removed the SDL2_LIBRARY_TEMP variable. Previously this held the path to the SDL2 library, which was used to construct the SDL2_LIBRARY variable, which contained other flags and libraries required for linking to SDL2. These other flags and libraries are now in the appropriate properties of the imported targets, so SDL2_LIBRARY can be the library itself again.
• SDL2main is no longer linked on the basis of the variable SDL2_BUILDING_LIBRARY; it is its own target SDL2::SDL2main, which can optionally be linked to.
• The addition of the -mwindows flag that selects the Windows GUI subsystem for MinGW has been moved to src/CMakeLists.txt. It used to be added in FindSDL2.cmake due to some faulty logic - it is required only for SDL2main, not SDL2, but was nevertheless added unconditionally. Now that SDL2main is a separate target, I decided we shouldn't be adding this flag as a side effect of using a library that doesn't need it, and instead add it explicitly in the relevant CMake file. (I'm not using CMake's WIN32_EXECUTABLE property yet, since that requires extra jiggery-pokery for MSVC which is out of scope here.)

Although I have given SDL2main its own target, it may be preferable to remove it entirely, since we don't use it and so I haven't tested that bit of code.

Only tested with MSVC and MinGW so far, let's see what happens elsewhere...

[DomClark]

It seems we only use SDL2 with MSVC in CI. If anyone uses SDL2 on Linux or Mac, it would be great if you could build and test this PR yourself.

The MSVC build is succeeding, but fails to find SDL2d.dll when packaging. I'm trying to figure out how to debug this.

[Veratil]

I believe my VM has SDL2, I'll test this next chance I get.

[rdrpenguin04]

I'm testing on Manjaro now

[rdrpenguin04]

Works on Manjaro, confirmed.

[Veratil]

Tested on Fedora 32, SDL2 is found and configured. Works when setting audio interface to it, too.

[DomClark]

CI passes now, and this has been tested on Linux and Windows. Now it needs reviewing, and I'd still like it tested on Mac (although if necessary we can merge without, and fix any problems that come up when we bring SDL2 to CI Mac builds).

[PhysSong]

Confirmed working on macOS.

[PhysSong]

Looks good to me except the question above.

[PhysSong]

Merge?