Add an export function to the keyboard shortcut setting panel

[gautierchomel]

Thorium settings panel displays the shortcuts list and allows users to modify them to his needs. It is authored with h3 titles.

Aside from this, for quick reference, the support page lists the default shortcuts and adds a short description. It is authored as a definition list instead of headings.

I have a hard time maintaining that second source. We could consider using the original content generated by Thorium to publish the list parallelly on the support page or enhance the Thorium list with definitions and an export (as HTML) button so the user can generate his helper/cheatsheet for quick reference.

edrlab/thorium-reader-doc#172