* Update AppStream data to modernize use of properties and add branding
colors.
* Update screenshots so they should follow Flathub's guidelines, and
display it running under KDE which is what is mainly used in
melonDS's development on Linux.
* Update manifest to refine permissions use and update the SDK.
OpenGL used to not work with the aarch64 KDE flatpak runtime, but this
seems to no longer be the case, so let's re-enable aarch64 builds.
There's currently an issue with gcc breaking rendering on aarch64 when
building with LTO, so it is disabled for now.