tsm: screen: implement selection extraction

tsm: screen: implement selection extraction

This adds a new helper that returns the UTF8 encoded data of the selected
parts in the tsm-screen object.

There is still much to do and it isn't a nice solution. However, it's a
proof-of-concept and works for now so we can just keep it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>