Spinner widget, event stealing, async "loading screen", Widget trait doc

[dhardy]

Adds a Spinner widget. Also, spot the new theme colour buttons in the Gallery. (This is with 150% scale factor.)

The usual controls are implemented:

• Text editing (immediate change in background on error, but only updates value on focus lost or Return key)
• Up/down arrow, mouse scroll
• Buttons on right, which are not keyboard-navigable but which do highlight on hover (though this is not very clear, depending on the theme colour)

To implement up/down and mouse-scroll event handling I added a new method to Widget:

/// Potentially steal an event before it reaches a child
fn steal_event(&mut self, mgr: &mut EventMgr, id: &WidgetId, event: &Event) -> Response;

This is functionality which was lost when SendEvent::send was removed; at the time there was not a good rationale for keeping it, but this widget does provide one: the ability to use a complex inner widget like EditBox (~1100 lines of code), but customise handling of specific events.

Documentation of the Widget trait (and co) was improved.

Additionally, the async-event example now shows a "loading..." message for 1s before async event handling starts. This is just a hint to show that this kind of thing is possible; a larger example might use the Stack widget to switch between the loading message and finished view (or just disable content and use a Progress mouse cursor).