[Qt] Prevent balloon-spam after rescan

`importaddress mrhz5ZgSF3C1BSdyCKt3gEdhKoRL5BNfJV`
adds like 2500 transactions to the wallet, this caused my desktop-panel to freeze, because of the balloon spam,
so show maximum of 100 balloons after rescan.
I'd suggest to make this name more descriptive of what is happening, instead of how the view should react to it. For example, processingQueuedTransactions().