Fix BasePriceEditor markup

Put markup on proxy_widgets. With this we don't have to update the model by
hand and kiwi will validate the widget for us, avoiding .read() raise a
ValidationError.

Also, remove self from run_dialog to get the right toplevel.

Change-Id: Ia5621dbb867957dd3486ab954c6e4c08c07254c0
(cherry picked from commit 115e42b48fc181ffd8c35a322d03331e0ad376ba)