Improve keyboard support

Issue description

It should be possible to focus the main menu with the keyboard shortcut Alt+Shit+Space, repeating that should move the focus around the parts of the respective Topincs component. The admin menu should open up with Alt-Shift-X, replace the current existing keystroke for index. Mouse move should not close it, if the keyboard focus in the admin menu. Ideally the admin can work without a mouse.

Developer comments

Sequence of focus jumps is by default :main menu, page menu, content. Until the first success. In the form it is: main menu, current form, page menu.

After a week of working with this. It works ok, but there needs to be some improvement: * The focus leaps by pressing Alt+Shift+Page down/up should work smarter and on any page with headlines: jump to the next headline, walk up the dom to look for focusable, then look below some ancestor. * If above works better it could be considered to reduce the focus jumps to menus.

The page is composed of larger parts which form a focus ring which is navigated by Alt-Shift-Space. It would be neat if the active element within a part is remembered. The numer of parts is usually header/main menu, content, with an optional page menu. In case of information resources it is many more parts.

The secondary navigation via up/down is a ring within the content roughly made up of the headlines.

Works really well with the latest improvements. A little bit more fine tuning necessary.

Works very well for modelling.