[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 say lower this to 10. 10 balloons at once is already a crazy party.