Add ability to specify fiat amount when sending bitcoin

Send in Fiat:
On Send Screen, user can press new button to specify amount to send in fiat (USD, EUR, etc.).  This will show a popup window that prompts the user for the fiat amount.  When the user clicks OK, it converts this amount to bitcoin and updates the existing bitcoin amount on the Send tab.  This feature only works if the exchange rate plugin is enabled and uses whatever currency the user has selected in the exchange rate plugin.