Add/improve: ComboBox + Spinner widgets

[dhardy]

ComboBox (drop-down menu) already exists, Spinner would be a new widget:

• When closed, mouse-wheel over the widget should change the active entry.
• Some type of arrow should be displayed indicating this is a drop-down menu. This should be theme-able, thus should depend on SizeHandle to allocate space and DrawHandle to draw it. (New Component trait; make Layout::size_rules optional; better layout for menus #305)
• A new "spinner" widget should be added for numeric entry, with up and down arrows. Likewise, these arrows should be drawn by the theme. (Spinner widget, event stealing, async "loading screen", Widget trait doc #319)

[dhardy]

(2) was implemented by #305.