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.