ComboBox virtualizing is disabled
If a control of type ItemsPresenter is used as item presenter, then the virtualization of the ComboBox is being disabled. Use VirutalizingStackPanel instead and set the property IsItemsHost to true.

Conflicts:
	MahApps.Metro/Styles/Controls.ComboBox.xaml
New features:
* Horizontal scroll bar: Show the scroll bar when panorama scrolling and wait a second to hide
* Mouse scroll: Use mouse wheel to scroll panorama
Both are independently and can be seted by xaml
Added a threestate CheckBox to the demo
Panorama dynamic group header - Use property DynamicGroupHeader of Panorama control to set behavior
Update PanoramaGroupHeaderConverter.cs

Change default margin
Update Panorama.xaml

Change default margin
Adjust project file

Ajust project file to include PanoramaGroup header converter
Added vertical sliders to the demo project
hwhoops this is kinda important
Implemented a FlipView control.
Added a style for DataGrid
Updates to DataGrid style including styles from stefan-schweiger. Update
to match Azure DataGrid styling.
Update column header style. Add disabled style.
Update DataGrid styles to use Gray brush resources instead of color resources. Make the default DataGrid style more like stefan-schweiger's. Add Azure DataGrid
style as a separate resource.
added control to demo
missing resource
dropped AzureDataGrid style while working out what to do with it
Calendar respecting dark/light theme
Reverted change to Virutalizing ComboBox because it breaks grouping

Last change set the itemspanel to virtualizing but this breaks the grouping behaviour of the combobox
fix flickering Issue #419

fix the nasty flickering at window starting.