Changed TextElement in HeaderTemplate to ContentPresenter, in order to allow e.g. Grid, StackPanel as child elment for the header of an Expander control.
Changed TextElement in HeaderTemplate to ContentPresenter, in order to allow e.g. Grid, StackPanel as child elment for the header of a GroupBox control
Merge pull request #875 from xxMUROxx/hotfix/ContentPresenter-In-Expander-GroupBox

Changed TextBlock to ContentPresenter in HeaderTemplate for Expander and GroupBox
add key for menu item style
Merge pull request #884 from MahApps/877-punker76-vs-menu-item-key

add key for menu item style