Introduce "inverted" branch insets: These are branches whose content

Introduce "inverted" branch insets: These are branches whose content
is output when a branch is NOT activated. Fixes bug #7698.

At the moment, inversion is controlled through the branch settings
dialog. There is no provision for inserting inverted insets directly,
or for changing them from the context menu. Both of these could be
done, of course. The latter would need LFUN_BRANCH_TOGGLE_INVERTED.