panel-plugin: migrate GtkTooltips to new simpler API.

In gtk+-3.x, the GtkTooltips struct is gone and you're supposed to use
the newer API that was available back in gtk+-2.x:

  https://developer.gnome.org/gtk2/stable/gtk-migrating-tooltips.html

For us, this is pretty straightforward: instead of declaring a GtkTooltips
member and doing stuff to it, we just call the gtk_widget_set_tooltip_text()
function on our plugin's eventbox instead.