Added keydown and focus handlers for Slot Section Menus ("Core Internal", "Optional Internal", etc.)
When focus is on the header, Enter key will open the menu and set focus to either the first option, or else the currently selected option, such as "Planetary Explorer" in Core Internal menu (if one has been previously selected).
While menu is open, Tab and Shift-Tab will move the focus up and down as expected. Shift-tab on first option will move focus to last option in the menu, and Tab on the last option will move focus to the top. Focus will stay inside the menu until menu is closed.
When focus is on a menu options, hitting the Enter key will trigger the onClick function for that option, and will set the option as the currently selected option for that menu.
Esc key will close the menu and set focus to the menu header H1 element.
