optimize multiline entry

Hi,
your frame implementation is not very nice, since you draw the frame yourself. You should use a native frame to keep the native feeling and honor users theme settings.
your change breaks this feature for PasswordEntry and SearchTextEntry.
I've added specific extensions methods for Gtk.Entry and Gtk.TextView. RenderPlaceholderText should not be applicable on other Gtk widgets (Gtk.Widget) so lets make it private.