Fix crash in on_socket_created() when password storage is enabled

Closes #164

See merge request !43

Fixes #164

See https://docs.gtk.org/gtk3/method.Menu.popup.html

See https://docs.gtk.org/gtk3/method.TreeView.set_rules_hint.html

According to https://docs.gtk.org/gtk3/class.AccelGroup.html, this is
setup automatically by GTK.

The buttons are now somewhat less pretty. Well, I don't know how to do
better.

Fix behaviour of : and :: in Scorefile and also make them match slrn documentation

Closes #158

See merge request !42

    IMPORTANT NOTE:

    This commit may break existing score files by changing the parsing
    rules to match slrn specifications (https://slrn.info/docs/score.html)

    Specifically, : means all of the rules must match to apply a score,
    and '::' means any of the rules can match to apply a score.

    Prior to this, they were the other way round, and Score:: didn't work
    at all (though {:: did)

    Refactored the loop and made the : and :: behave according to the manual

Fix automatic marking of articles read crashing in an infinite recursion.

Closes #156

See merge request !41

This method was hollowed out by previous commit

Looks like the goal of this method was to display a greyed out "Group"
inside the group search widget when this one was empty and not in
focus.

Knowing that:
- this feature has not worked for a while (as far as I remember)
- this feature is not really useful (a popup balloon would be more
helpful but I don't know to write one in Gtk)
- this function calls deprecated gtk_widget_override_color

I think it's better to remove it (and the other call to
gtk_widget_override_color that was supposed to put back normal color).

fixup grayed out

mostly a cut'n'paste from http://pan.rebelbase.com/mailing_lists/