[Qt] re-work overviewpage UI

- ensure normal and watch-only stuff looks consistent
- simplify UI by removing unneeded UI layout elements
- change some comments to watch-only from watchonly
Merge pull request #4627

94e1b9e [Qt] re-work overviewpage UI (Philip Kaufmann)
[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