GUI feature "fancy" To-field

Features:
* combobox with autocompletion for labels from YourIdentities/AddressBook/Subscriptions
  - accepts multiple labels separated by comma or semicolon with stepwise display of details
  - also accepts addresses for mixed multiple entries, e.g. "BM-...,[chan] general,BM-..."
* combobox with autocompletion for addresses from YourIdentities/AddressBook/Subscriptions
  - accepts multiple addresses separated by comma or semicolon with stepwise display of details
  - also accepts namecoin addresses: either push the button manually or put "id/" in front of it
* scroll through recipient addresses and delete one or more of them from the To-field
* "chanonymous" button to send from the same address as selected
  - inversely, "send to chan" to add the current From-address as a recipient

Just like before:
* doesn't allow sending to yourself except for chans
* send to address from Address Book

Future:
* may be enhanced by Identicons and avatars