Windows Support

[JFronny]

This PR adds the necessary bits to make creating cross-platform applications using java-gi easy, as is demonstrated here.
(Maybe that repo should be referenced in the README or cloned to your account for visibility?)

Changes to java-gi

• To support the different names for natives on Windows, a class handling the loading of libraries is implemented in LibLoad (this name can, of course, be changed)
• A manual CI task that builds GTK on GitHub actions and uploads the resulting binaries to a release titled natives is added.
  These natives can be used in applications instead of self-built ones
  In order to run it, select the task in the Actions tab on GitHub, click Run workflow, enter the GTK version to build and whether to add the binaries to the natives release, then run the workflow.
  It is never executed automatically.
• The regression mentioned in Register types to fix casting #27 is fixed, allowing this to actually work

Features of javagi-multiplatform (the example project for these changes)

• This example application is modular
• An installer for Windows is built and contains the natives generated by the abovementioned tasks, so the user simply needs to install the application using it (and the msvc++ rt, though they likely already have that) for it to work
• The generation of said installer (and linux packages) is automated through GitHub Actions
• Theoretically, there's also a MacOS package, though that probably doesn't work (and I don't really want to figure out another platform)

[jwharm]

I don't have a Windows machine to test it on, so I did some regression testing and it works fine.
Very happy with the fix for modular applications too.

[jwharm]

Please be aware that you will need to account for the different sizeof(long) on Windows vs Linux. As it is, the generated memory layouts assume sizeof(long) == 64 so accessing struct fields on Windows will be problematic.

[JFronny]

I don't have a Windows machine to test it on

Setting up a quick Qemu VM should be enough for testing (that's what I do)