closedcaption: implement tttocea608 element

Marking as WIP because this is my working branch and it contains patches from !280 (comment 441254), I need them for testing locally.

This only implements "pop_on" mode, it could be extended in the future to support either roll-up or paint-on modes as well. It doesn't implement any sort of text layout, and always displays the received text as is, clipping it after 2 rows and 32 columns. A separate element that would perform splitting up / text layout would be pretty nice IMO, but that's a separate discussion.