Expander enhancements

[punker76]

• some styling fixes (margin, padding, border thickness)
• add ExpanderHelper with
  • 4 new attached dependency properties HeaderUpStyle, HeaderDownStyle and HeaderLeftStyle, HeaderRightStyle. so it's now possible to create your own header toggle styles without rewriting the Expander style
• better handling for BorderThickness
  • the Expander uses now the new value converter ThicknessBindingConverter, so setting the BorderThickness is much better than before
• Click on Expander header support Click on Expander header support #1291 (just do the default expander behavior)
• new PreserveTextCase attached property

and for GroupBox

• some styling fixes (margin, padding, border thickness)
• PreserveTextCase attached property
• ThicknessBindingConverter, so setting the BorderThickness is much better than before

[punker76]

^ @xxMUROxx @Dotnetifier it's time for the click on header behavior :-D