Change recommended replacement for gtk_show_uri()

Problem is GtkFileLauncher is unable to handle all the types of URIs that are supported by gtk_show_uri(), e.g. help: URIs. GtkUriLauncher avoids this problem.

Another problem is that GtkUriLauncher is just generally a better choice for launching URIs, since you don't have to create a GFile in order to use it. Porting code is slightly simpler.

The documentation still mentions both GtkFileLauncher and GtkUriLauncher as options, but most people will use whatever the compiler recommends when it prints the deprecation warning.