Webclient: keyboard control for menus (#534).

All menus can be controlled via keyboard.
Taken from jQuery UI website:

- ENTER/SPACE: Invoke the focused menu item's action.
- UP: Move focus to the previous menu item.
- DOWN: Move focus to the next menu item.

Typing a letter moves focus to the first item whose title starts with that character. Repeating the same character cycles through matching items. Typing more characters within the one second timer matches those characters.

Disabled items can receive keyboard focus, but do not allow any other interaction.

See #534.