MenuDialog: Redo delegate, improve typing

Since this top-level component exists solely for its internals, we
assume that the column and header are not replaceable, hence we can rely
on IDs and use more robust alias property for header's content item.

In delegate, almost every binding has changed. Most properties are now
automagically bound through AbstractButton::action. Marking context
property as required allows getting rid in context injection and add
explicit typing.  Setting padding in this new way saves one binding
expression, thus should evaluate faster with native C++ padding
fallback chain. ToolTip now uses built-in pressed/hovered properties of
the list item itself rather than stacking an extra HoverHandler on top
(and not accounting for tablet mode, apparently).