Add Flatpak

[ReillyBrogan]

• Change appId on Linux to dev.twilitrealm.Dusklight
• Install assets with cmake
• Fix platform name (was getting set to "cmake_system_name")
• Add flatpak

Part of #1862

[encounter]

Maybe we conditionally set this to .Dusklight for Linux builds and use this variable downstream instead of LINUX_APP_ID. We can also include it in our version.h.in so that SDL_SetAppMetadata can use it directly rather than the conditional logic

[ReillyBrogan]

I've taken that a little bit further. The app_id is now configurable and defaults to dev.twilitrealm.Dusklight on Linux and dev.twilitrealm.dusk on all other platforms. With DUSK_PACKAGE_INSTALL all files installed use the app_id as part of their name or install location and so it should be possible for forks to change the app_id to be co-installable without any further change.