The filter menu button in a RibbonGallery has a different default Style (with a much different Template) than a normal RibbonMenuButton. Therefore, we've decided to create a new class for it so that we can specify our Style as the default. This way, if the user specifies RibbonGallery.FilterMenuButtonStyle, the supplied Style is merged with our default Style instead of throwing it away. Thus, app authors don't have to retemplate the filter menu button if they just want to restyle a few properties.
