Change recommended replacement for gtk_show_uri()

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.