Give LastComposedWord knowledge of the separator (A2)

This stores the separator that was used to commit the word in
the LastComposedWord. It may be NOT_A_SEPARATOR if there was
no separator (for example, the cursor moved causing a commit,
or there was a manual pick). This is necessary to implement
feature request #5968922.

Change-Id: I5fcf19a78ec66d68d4df89418eaef13952588207