Make status always available during startup, e.g. toggling splash screen, or systray tooltip #4644

On Windows, while starting Bitcoin-qt after placing an old wallet.dat in my data directory, I recently lost the splash screen and had no status updates for 20 minutes: #4642

I should always have a way to know what the program is doing, especially during a lengthy operation.

One fix would be to allow me to bring the splash screen back when I click or right click on the system tray icon.

A second fix would be to display the status messages from the splash screen as tooltips when I hover over the systray icon.

Could be seen as a duplicate of #3282, but I'm keeping this one as it provides a larger number of possible solutions.

@laanwj I also thought about this for a little while and perhaps we could init the tray icon in SplashScreen and attach an onClicked event to ->show() it again, dunno if that works. But we have to make sure that splash-tray and gui-tray don't interfere with eachother.

@Diapolo Moving the tray icon up to BitcoinApplication and adding a createTrayIcon method which is called before creating the splash screen would make sense.

Apart from that, implementing "click on the tray icon and re-show the splash" is mostly plumbing to make sure that

• The splash is not immediately deleted when hidden
• Make sure that the 'clicked' signal of the tray icon ends up at the splash screen, and triggers a re-show
• Possibly: ->setToolTip() on the tray icon on init messages
• As the splash screen should be deleted before the main GUI is shown, in principle the signals should never interfere with each other

@laanwj I'm not going to have the time over the next days/weeks for this... are you going to take a look at implementing the changes? If not I could start with this in ~3 weeks.

Solved by #4941