Bitcoin-Qt: use statustips in addition to tooltips

• 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"