Make the popup dialogs work (canvas), with a demo page. Fix exit issue.

[cmeyer]

• Fix the select item popup with no items or an invalid current item.
• Size the select item popup and its list to the items rather than to a fixed width.
• Add a popups demo page to the ui demo application.
• Do not register a window without a parent as a dialog of the application.
• Create a child canvas window under the window of the host user interface.
• Pass the host window close along to the canvas window which wraps it.
• Take the keyboard focus of the host when a canvas widget is given focus.
• Keep the width asked for when a canvas line edit sizes to its text.
• Make the edit string popup field fill the popup.
• Do not ask the host to quit from within a window close.