Change IdentityChooser to allow keyboard searches/usage in write window

Phenomenon

If a large amount of identities is configured in YAM and a new mail is being composed by a user it takes a while to find and select the desired identity in the identity chooser cycle gadget. Therefore a fast and easy way seems to be missing to quickly select the identity using the keyboard.

Background analysis

A standard cycle gadget only provides a plain list without any possibility to quickly select an entry using a keyboard search. It should be possible to activate the identity chooser gadget and enter characters in the keyboard to quickly search and select the right user identity.

Implementation recommendation

It might be possible to simply use a string gadget with a cycle gadget alike background and frame. Then when entering some text or selecting the gadget it should popup a list with the identities and the current match should be selected. If enter is pressed the active object should be advanced to the next one (e.g. the To: string gadget).