Selectmenu: Cleanup

Ref gh-1224
Selectmenu: Don't use active items at all

Menus only use active items for nested menus which selectmenu doesn't support.
Selectmenu should only be working with focused items.

Ref gh-1224
Selectmenu: Simplify selection of focused item via keyboard

Don't go through menu to select the currently focused item.

Renamed _selectMenu() to _selectFocusedItem() for clarity.

Closes gh-1224
Selectmenu: Remove unused var