Remove grouping from ComboBox and make GalleryPanel inherit from StackPanel

[batzen]

Grouping in ComboBox is broken for roughly a year now, after i changed the GalleryPanel's inheritance from Panel to VirtualizingStackPanel to fix #142 with commit c6ccc78.
It's also noted in #305 the there are massive problems with the ComboBox currently.
Because no one seems to have noticed/missed it I will remove the grouping feature from the ComboBox entirely and switch GalleryPanel back to inherit from StackPanel instead of VirtualizingStackPanel.

Switching to VirtualizingStackPanel in GalleryPanel back then was a bad idea as the GalleryPanel introduces it's own items when rendering which causes a lot of problems when using a VirtualizingStackPanel as a base class.

By removing the grouping from ComboBox i can also completely remove the dependency on GalleryPanel from the ComboBox any rely on the standard ItemsPresenter.
This also allows for changing the ItemsPanel of ComboBox thus making virtualized/non virtualized ComboBox possible.

It's still possible to use GalleryPanel as the ItemsPanel for the ComboBox. To make it work you just have to:
On the ComboBox:

ScrollViewer.CanContentScroll="False"

As itemspanel:

<Fluent:ComboBox.ItemsPanel>
    <ItemsPanelTemplate>
        <Fluent:GalleryPanel GroupBy="Tag"
                                ItemContainerGenerator="{Binding ItemContainerGenerator, RelativeSource={RelativeSource FindAncestor, AncestorType=ComboBox}}" />
    </ItemsPanelTemplate>
</Fluent:ComboBox.ItemsPanel>

@GeertvanHorrik @robdg80 you might be interested in this breaking change.