Add manifest for building as a flatpak

[cpba]

Includes the following:

• org.DolphinEmu.dolphin-emu.metainfo.xml.in this file contains general information and metadata that is used by most linux based app stores (kde discover, gnome software, flathub.org, etc) to showcase dolphin.

• fill_release_node.sh a script to automatically fill in the release information in the metainfo file when building the flatpak.

• SDL2.json manifest to build the vendored version of SDL from the Exports directory, it's only temporarily needed until the kde runtime is updated with the current SDL2 version and should be deleted after.

• org.DolphinEmu.dolphin-emu.yml the manifest itself.