flatpak: Simplify cores

Libretro people don't use make install, and the install rules are unmaintained. They frequently break, and there have been cases where they were just removed during cleanups.

Not all cores have the install rule either. For those that don't, we inject custom makefiles that add one. The arguments you pass to specify the installation prefix also vary per core. And some cores are built with CMake, and for those, we just install the sofile in post-install.

In addition, we also always use post-install to install core descriptors.

Overall, this is very complicated. Instead, we can drop all of this and install every core in post-install together with its descriptor. It's both simpler, less fragile and doesn't involve makefiles, even if it's less clean in theory. The number of changed lines speaks for itself here.

Stop putting descriptors into subdirectories. While it shouldn't work, Builder doesn't like it, so desmume, dosbox and nestopia weren't getting built.

While we're here, bundle the Gearsystem descriptor file. While the original idea with these files was to push them upstream, by now it's safe to say that it won't happen:

• nobody except us uses core descriptors;
• we'll need to mass update those files later when adding touch controls
• we need them to be translatable to display firmware in the UI

With this in mind, not only it's better to have them in tree, it's better to have them on the app side and not packaging side, so that's where we'll move them later.