[Qt] ensure all class attributes are init to 0

- in BitcoinGUI and UnitDisplayStatusBarControl
[Qt] use BitcoinGUI::DEFAULT_WALLET constant in bitcoin.cpp
[Qt] disable tray interactions when client model set to 0

- this prevents the ability to fiddle around with the system tray when
  already shutting down (e.g. on slow shutdowns because of a proxy delay)
- extends solution for #4360
Merge pull request #4649

b197bf3 [Qt] disable tray interactions when client model set to 0 (Philip Kaufmann)
314fbd9 [Qt] use BitcoinGUI::DEFAULT_WALLET constant in bitcoin.cpp (Philip Kaufmann)
8ca6a16 [Qt] ensure all class attributes are init to 0 (Philip Kaufmann)