Fix transaction fee in uBTC

Step for buttons 'up' and 'down' - 0.001. With BTC and mBTC all ok, but
0.001 uBTC is lower than minimal value (satoshi)
User should press 10 times on 'up' button to get 0.01 uBTC
Save & restore window size and position
Show window in center of screen on first launch
Merge pull request #2393 from r000n/staging

Staging - GUI fixes for upstream