Add support for appending/prepending hidden words in the annotation editor

[proycon]

One can currently add new words/tokens by editing the text of an existing word. This functionality could be expanded to allow for adding hidden words as well.

[proycon]

This is a bit of an advanced feature that may be not be evident. Insertion (append/prepend) of normal words is done by getting the token in front/after and changing its text by simple adding or prepending another word (i.e. space separated). The key to make this word a hidden word instead of a normal one is to enclose it in tildes, like so: ~word~.

Any more complex edits of structure is reserved for the future structure editor (#5).