GitLab

- add setStatusTip() in addition to setTooltip() where it makes sense
- add only setStatusTip() if GUI element is only used in main- or tray menu

- add an event filter on our BitcoinGUI object to prevent garbelled text
  on the status bar, which happens when we use it for e.g. displaying
  block-sync state and then a QEvent::StatusTip wants to write own text to it

- remove a double translation of "Bitcoin client"