GtkLabel: don't drop PRIMARY selection when visible selection is cleared

We must remember the last selection positions, in case the selection is
cleared.

https://bugzilla.gnome.org/show_bug.cgi?id=584236
GtkLabel: clear PRIMARY selection when the text changes

When the text changes, the previously selected text is generally not the
same anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=584236
GtkEntry: don't drop PRIMARY selection when visible selection is cleared

Same modifications as in GtkLabel.

https://bugzilla.gnome.org/show_bug.cgi?id=584236
GtkEntry: update last selection positions when text changes