Add highlighting methods to the TextFragment trait.

Add highlighting methods to the TextFragment trait.

I've chosen to have three rather than two classes of thing you can
highlight, by separately indicating whether a text fragment contains
highlightable 'statuses' or 'whole statuses'. The distinction is that
a 'whole status' means you can see all the text, whereas a 'status'
just means you can somehow identify which post it is. I'll use that to
arrange that replying, favouriting and boosting only apply to whole
statuses (too easy to make a mistake otherwise), whereas any 'get
more info' commands apply to any old status.

(In particular, this way, you can see a one-line summary of a status,
press [I] to look at the whole thing, and _then_ fave/boost/reply.)