Improve lexicon submenus: dynamic chevrons, hiding

Submenus now hide themselves properly when clicked on, and the chevron
points up or down to indicate whether the menu will be shown or hidden.
Improve submenus: hide when choice is made

Submenus should also hide when user clicks on a choice.
Improve submenus: correct mouse pointer, underline

Submenu "links" (that is, the word clicked on to show the submenu)
should use the "hand" mouse pointer, but not show an underline when
hovered over because they don't go to a new page. The links that do go
to a new page, though, should still show an underline when hovered over.

With this commit, the submenus look the way I want them to look.