Revert "Automatically show the review toolbar if the document has tracked changes"

Revert "Automatically show the review toolbar if the document has tracked changes"

We will replace this with a better solution

For now, only keep
- Changes::isChanged()
- Buffer::areChangesPresent(), replaced by a dummy function

Next step will be to provide a working areChangesPresent() and to
compute Inset::isChanged in updateBuffer.

This reverts commit 6d4e6aad24edb7bcfbc49f03d2432fc9fa06954d.