Add AppImage Packaging script

[GT190]

This adds a script, which builds a Dolphin AppImage. It is intended to be used with Dolphins CI, but also works when locally compiling Dolphin with these cmake flags: -DLINUX_LOCAL_DEV=true -DCMAKE_INSTALL_PREFIX=/usr -GNinja. It needs to be executed from the Build directory, which can be done with ../BuildLinuxAppImage.sh.

The script installs Dolphin into a folder called AppDir, adds the translation files into the AppDir and builds an AppImage from the AppDir folder.

Necessary for dolphin-emu/sadm#198

[OatmealDome]

Just a quick note: I haven't forgotten about this PR. I will be touching the CI infrastructure in the near future to add Flatpak support, so I believe it'll be easier for me if I do this at the same time.

[Samueru-sama]

Hey what's holding this?

The PR looks good, I'm surprised that it even uses the right appimagetool to make the AppImage with the static appimage runtime which has no libfuse2 dependency.

[OatmealDome]

Thanks for testing. This requires some CI infrastructure work before it can be merged. I'm currently working on Flatpak infrastructure and will move on to AppImage after that is done.