Fake middle clicks without gtk_button_{press,release}, which are deprecated

Factor the logic that fakes clicks from a middle click in
EphyMiddleClick(Tool)Button by forwarding a left click to GTK+ when we
receive a middle click. Since ephy_gui_is_middle_click stops working
in this case, add the minimal logic in EphyLinkAction to make it work
again (basically, cache the button that activated the action inside
the action itself).

The EphyMiddleClickable(Tool)Button classes were written by Alexandre
Mazari.

https://bugzilla.gnome.org/show_bug.cgi?id=628364