Add onfocus=, onunfocus=, and focus() event to widgets.

[TheOlajos]

This will help streamline and make event execution more controllable. Especially on items like input where events are executed currently onchange or in an eval or trigger. This will also allow a user to set the focus of a widget to true or false.

• add a non settable focus attribute so a user can check if a widget has focus, or a hasfocus() event?
• Add a focus() event that can take an id, next, previous, or none.

[BrayFlex]

It would also be nice to add a focusnext="WIDGET_ID" and implement: https://docs.flutter.dev/ui/advanced/focus#focustraversalpolicy
so we can guarantee proper TAB traversal of fields.