flatpak: Rewrite noenumarate=true filtering code

For apps installed from repos that have noenumarate=true, we were using the origin name (e.g. "org.frozen_bubble.frozen-bubble-origin") and tried to figure out the installed ref based on that. The code would split on - and append .desktop and end up with something like "org.frozen_bubble.frozen.desktop", which is pretty far from the real app name "org.frozen_bubble.frozen-bubble".

This commit replaces this by just asking flatpak what ref is installed from that repo.

This is slightly made more complicated by the fact that the flatpak API only appears in flatpak 1.1.1, but we need to support 1.0 as well, so this commit also adds fallback to parsing the key file manually.

Fixes: #195 (closed)