Annots: partial revert of commit 0b474ffb to except form buttons
The original commit unconditionally creates a new appearance and thus discards the previous selection state of a checkbox. As a result, only one checkbox could be selected at a time.
Given the motivation for commit 0b474ffb was to affect free text annotations, we here restore the original behavior for any formButton.
Fixes:
Issue #1504
https://gitlab.gnome.org/GNOME/evince/-/issues/2043