label: Use GtkFileLauncher for file URI
The GtkUriLauncher calls into the openuri portal, which distinguishes between files, directories, and URI. The GtkFileLauncher contains logic to deal with this, because it can already handle the file and folder differences.
If we have a file:// URI it's easier to create a GFile out of it, and use the GtkFileLauncher API, while leaving the GtkUriLauncher API for every other URI scheme.
Same fix as de3c1d0c, for GtkLabel.
Fixes: #5671 (closed)