Fix linux (gnome shell) icon and notification grouping

This branch contains fixes for Gnome shell including the following:
* fixes irssi icon temporary folder path
* replaces use of "notify-send" command with use of a conditional import of the GTK gi.respository.Notify api
 * implements use of categorization and groups messages together under the name "irssi"

For Linux window managers other than Gnome Shell, this may break notifications entirely, but I have not tested.
I also have not updated the command-line help output.