custom frameset restore

[gagbo]

Fixes #41

[gagbo]

I'm not 100% positive that the documentation in the defcustom matches earlier versions of Emacs, I still need to check that

[alphapapa]

Ok, please let me know when it's ready. Also, please lint with makem.sh, e.g. Checkdoc will complain about those docstrings. And please put the nil initial values on the same line as the variable name.

[gagbo]

Sure I'll lint with makem.sh once I checked frameset-restore on 26.3 and 27.1/2; I already ran checkdoc so I wonder if it's going to flag other things in docstrings

[gagbo]

Sorry I’m a little busy, hopefully I’ll find some time to finish it soon TM.

I had time to check the API of frameset-restore on emacs-26 and emacs-27 branches, and it’s strictly the same, so the only step left is to lint with makem.sh

[gagbo]

I passed makem, but I had issues with spellchecking, not sure it's passing