This PR adds a SplitButton to the MDIX library by introducing a SplitButton custom control which is a composite control consisting of a normal Button and a PopupBox next to it. It tries to leverage the existing button styles to avoid too much maintenance work.

PROs:

• Discoverability (independent control residing in the MDIX namespace)
• Hope to have fine grained control if tweaks are needed that should not affect normal buttons (may need some work though!)
• Does not impact the visual tree of normal buttons

CONs:

• Some level of duplication is still present, but not a whole lot

NOTE: I opted to not use the new cool resource key stuff generated by mdresgen yet. It will probably require changes in the Button templates as well, and as such I would rather get this in and then open a new PR where I can try to use the new features of the theme generation.

Demo apps are extended with the following (on the Buttons page) showcasing all currently added styles:

Example usage is something along these lines:

<materialDesign:SplitButton Content="Split Button"
                            Style="{StaticResource MaterialDesignOutlinedSplitButton}"
                            PopupPlacementMode="TopAndAlignRightEdges">
  <materialDesign:SplitButton.PopupContent>
    ...
  </materialDesign:SplitButton.PopupContent>
</materialDesign:SplitButton>