Added exchange rate to status panel

Added current exchange rate to status panel after Balance. For example, " 1 BTC~734.55 USD". Uses settings from Exchange Rate plugin and behaves same way as existing fiat balance in status panel.

Also fixed bug with width of combo box on bottom right.

![electrumcombo](https://cloud.githubusercontent.com/assets/6622335/2530822/feaeed68-b52a-11e3-9484-b125503228e9.jpg)

![electrumstatus](https://cloud.githubusercontent.com/assets/6622335/2530825/03ba527a-b52b-11e3-98dc-f9dc8f304373.jpg)