Requesting payment

[cmihai]

Is your feature request related to a problem? Please describe
The Unit-e Qt wallet allows "requesting payment" -- creating a receive address and a unite: URI for the address, potentially encoding the amount, comment and narration. The URI looks like this: unite:2MtVvZg9atGgSFL9uVF6yF5svn8A6MoMHbD?amount=1.00000000&label=Landlord&message=Rent%20payment%20December. The Electron wallet only allows you to copy the plain address.

Describe the solution you'd like
The Electron wallet should allow generating unite: URIs, encoding the amount and address book label (no narration, since we removed that feature). The wallet should also register itself as a handler for unite: URIs in the system.