screensaver-proxy: Better metadata for SDL games

SDL applications (usually games) don't have any API to provide an
application ID and a reason for inhibition, so if those are Flatpak'ed
get the application ID from the Flatpak, and use the gathered name as
the reason for inhibition.

https://bugzilla.gnome.org/show_bug.cgi?id=767982
6 jobs for wip/hadess/better-sdl-metadata in 2 minutes and 15 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #297520
build

00:00:43

 
  Test
passed #297521
test

00:01:31

 
  Manual
manual #297522
allowed to fail manual
asan
manual #297525
allowed to fail manual
msan
manual #297523
allowed to fail manual
tsan
manual #297524
allowed to fail manual
ubsan