Fix keyboard focus issues with a demo page for keyboard focus

[cmeyer]

• Report focus changes from a canvas widget to its focus changed callback.
• Add a focus changed callback to the declarative user interface.
• Report, take, and give up the focus of a list view widget.
• Report a list view return press only for the return key, not for a double click.
• Focus the first focusable canvas item when a canvas widget is tabbed into.
• Add a focus and keyboard demo page to the ui demo application.
• Make the tab key move the focus through a canvas item hierarchy.
• Let a canvas widget hold the focus on behalf of the canvas items drawn in it.
• Let a push button take the focus and the keyboard in the canvas backend.
• Let a check box and radio button take the focus and the keyboard in the canvas backend.
• Let a combo box and slider take the focus and the keyboard in the canvas backend.
• Let a check box or slider canvas item take the focus only when it is a control of its own.
• Leave the focus where it is when a control is clicked.