Display a header line when editing annotation contents

[astoff]

Instead of using an echo area message that disappears, display the keys to commit and abort in the header line. This is similar to what org-capture does.

I could also add a customization option to keep the old behavior (but note that a more advanced user can do this anyway using pdf-annot-edit-contents-minor-mode-hook).

Finally, a tangential comment: For consistency with org-capture and magit's commit message editor, pdf-annot-edit-contents-abort should be C-c C-k instead of C-c C-q. Should we add an additional binding in the minor mode map?

[vedang]

This change looks good to me! I will test it and merge it in tomorrow. +1 to adding C-c C-k as an additional key-binding to abort the edit. If you get around to it before I merge this in, just add a new commit to this PR, or submit a new PR.

Thanks!

[astoff]

+1 to adding C-c C-k as an additional key-binding

Done!

[vedang]

Merging this in, thanks for the change!