(TIR 16) support merging and splitting tokens

[balmas]

Originally stated as:

16. Must allow user to edit tokenization results before they start aligning a text (because supporting editing words once alignment is started is not in scope)

This is expanded to as follows.

Once an alignment has been created the interface should allow the user to do the following for any aligned text:

1. merge two tokens
2. split one token into two

If a token that is being split or merged already participates in an aligned group, the interface should warn the user that the existing alignment of this token will be erased if they proceed (and then do so if they continue with the action...)

[balmas]

additional requirement:

when merging or splitting tokens, the user should be offered the option to apply the action to all like tokens in the aligned text