linkbutton: Use GtkFileLauncher for file URI

linkbutton: 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.

Fixes: #5671