Qt Designer: Add customizable property tooltips.

Qt Designer: Add customizable property tooltips.

- Introduce tag propertytooltip to property specifications
  in the ui XML snippet returned by
  QDesignerCustomWidgetInterface::domXml()
- Pass the text to the property editor and set as description tooltip
  on the property.
- As an added benefit, show the type in the normal tooltip.
- Demonstrate using TicTacToe example.

[ChangeLog][Qt Designer] Added customizable property tooltips.

Task-number: QTBUG-45442
Change-Id: I371bbbb3a6f2bc0f433193b5eb45658211ca67de
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>