Navigation - help with keyboard shortcuts

[szabgab]

Problem

In anticipation of allowing book authors do define / enable more keyboard shortcuts and even without that it would be a good idea to make
it easy for readers to find out the available keyboard shortcuts. AFAIK the button to activate this is usually ?, though we might want to also add a help icon to the menu-bar.

See also #1408

Proposed Solution

Let's start with a simple overlay with some fixed text and then make it based on the local configuration.

Notes

No response