Allow ListBox to operate with both SELECTION_MULTIPLE and activate_on_single_click
Fixes #497.
Prior to this fix, GtkListBox acts very strangely when you enable both:
selection_mode = GTK_SELECTION_MULTIPLE
activate_on_single_click = true
Under this patch, the selection behavior now exactly matches GTK_SELECTION_MULTIPLE with the addition of emitting activation events whenever an unmodified click hits a row.
The fix for GTK4 is basically exactly the same, so if this looks good I can submit that MR as well.
Edited by djrenren