Widget geometry should be saved together with other Settings

[irgolic]

Right now widget geometry saving is tied to resizeEvent, hideEvent, moveEvent and closeEvent in OWBaseWidget.

Instead, it should use a mechanic like the one implemented in #151. After that's merged:

• widget geometry should be saved on settingsAboutTobePacked
• sync_node_properties should also be called when a widget's window is unfocused (intrinsically calls settingsAboutToBePacked)