Revive CMake support

It seems, at least on the Android front, quite some things have improved in terms of Multi-ABI support, see this blog post: https://www.qt.io/blog/qt-5.14-android-multi-abi-and-cmake

We could consider reviving the cmake build for the app, as this still somehow seems to be the better way forward. Ideally, however, this is also done in conjunction with moving the iOS build to cmake.

Step Targets

Currently, the app is supported on a list of target operating systems and architectures. Ideally, we could use cmake for all of them (and then do the final switch). The list is:

• Android
  • AAB, which is a super-set of the individual APKs for
  • armv7
  • arm64_v8a
  • x86
  • x86_64
• Linux
  • x86_64 AppImage
  • Flatpak
  • Snap -> will be handled via #417 (closed)
  • Arch Linux PKGBUILD/AUR
• iOS -> will be handled via #418 (closed)
• macOS x86_64
• Windows
  • x86
  • x86_64