Add multisig dialog.

This is a new dialog (accessible through an entry in the 'File' menu) allowing basic operations with multisig addresses (more easily than with the RPC system):
  - creating a multisig address
  - creating a new raw transaction (e.g. to spend the funds of a multisig address)
  - signing a raw transaction
  - sending a raw transaction to the network

I made some simple tests and it seems to work fine, but there are probably a few bugs still in there, so more testing will be necessary.

Also, I didn't spend too much time on the look and feel of the dialog, so there's room for improvement...