[#168] Allow for item transferral via actor sheet.

[krbz999]

Implements socket handlers for updating chat messages and creating items on other actors.

• A setting is added to assign the "Party" actor via id.
• A context menu option is added to "physical" items on actor sheets.
• Selecting a target to transfer the item to creates a chat message.
• The owner can cancel the transfer, which deletes the item, or the receiver can accept it.

Closes #168.