Add GitHub Actions workflow to build for all supported platforms

[jmbowman]

This doesn't yet sign any of the installers for platforms which require that for easy installation; I'll try to configure that next (only when the workflow is manually run for releases). Also made some other build-related fixes and improvements:

• Updated the Android build configuration to work with SDK 34 (Android 14), and target SDK 33 so it can be reactivated in the Google Play app store.
• Added support for building all Android processor architectures
• Updated the Windows build to use the Qt distribution of MinGW to also compile Metakit
• Updated all shell scripts to avoid continuing even after significant errors
• Finished support for including the (still very incomplete) Polish translation
• Removed hard-coded Qt versions and paths from everywhere other than the new workflow
• Updated some build instructions and comments
• Updated the copyright years

[jmbowman]

Status report for future reference:

• Ubuntu builds working!
• macOS 13 build is failing due to https://bugreports.qt.io/browse/QTBUG-117225 . There may be no practical fix for this short of upgrading to Qt 6 (I don't want to patch and compile Qt from source in a GitHub Action), but it shouldn't block the macOS 12 build.
• I have the wrong syntax for running a command in MSYS2 for the Windows build
• The Android build finished compiling, but ran afoul of problems in linking. Still need to investigate.

[jmbowman]

Another status update:

• Ubuntu and macOS release packages are now successfully being built (although the macOS package isn't signed for easy installation yet).
• The Windows build is successfully compiling, the current failure looks like a straightforward fix of correcting a path for the installer.
• The Android build is also successfully compiling, but isn't getting an appropriate version of gradle to process the Java side of things yet. I'll take a stab at debugging that locally, hopefully it'll only require fixing the gradle wrapper configuration.