Push &EventState into DrawHandle; remove DrawCtx

[dhardy]

Fixes:

• EditField: do not show cursor if not editable
• EditField: do not flash cursor if window is inactive
• ComboBox: use accent color when depressed or focussed

Changes:

• Push &EventState into DrawHandle and pass &WidgetId in methods, allowing calculation of event state in theme code as required
• Add CheckBox::Editable state
• Add WidgetId::parent()
• Remove per-widget disabled flag; instead add list of disabled widgets in EventState. Events targetting disabled widgets or their descendants will now automatically be sent to the parent of the disabled widget instead.
• Remove DrawCtx. Instead add IdRect and IdCoord allowing easy parameterisation of methods from any &W where W: Widget or &CoreData.
• Add kas_core::theme::Background enum passed to DrawHandle::frame