Add accelerators to page/extension menus

[eugenesvk]

Also added a few icons for much faster visual differentiation between the options

• 📝 Annotate and save the page
• Annotate and save the page

By the way, do you know how to force those text icons to be in place of actual icons in the menus (there is a dedicated column for them left of text, this would solve the misalignment issue)?

I'd gone with the tradition of adding accelerators to the matching letters within the name if found, but honestly I'd just move all of them to the left just like for &D Save selection" since it's (to me) more important to see the shortcut right away, and those tiny underscores in the middle of sentences aren't very functional even if looking nicer

Another downside is that the extension menu has those ugly & marks not removed in some browsers (not in Chrome, though), and even with them the accelerators seem to work, but that's on browsers

I'd also rename the extension to &SingleFile just for this :) but that's maybe going too far, will just do it locally (though would be nice to find a way to somehow make it user configurable not to do ugly local patches)

(moved items around to match the menu order to make keeping track of the shortcuts/edits easier)

Partially addresses #1420