Add .gitignore to m4/ and po/ directories

Instead of trying to keep up with files added by Gettext tools and
Autotools, maintain a list of tracked files in those directories.
Fix URI handling for GTK+ 2.24 and later

URI handling relied on GTK functions declared obsolete in GTK+ 2.24 and
later.  URIs are now handled by the desktop defined default/preferred
apps when either the Web browser or Mail program fields are blank.
Document notes of interest to development