Show the branch in the source popover

Now we're not just pretending that only the newest branch exists we need to
show the branch in the dropdown to avoid having identical entries. This new
string should only be shown for runtimes and extension points and not actual
applications as typically only one branch is available there.
1 job for wip/hughsie/flatpak-branch in 19 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #337060