Update FilteredSiteMapNodeVisibilityProvider.cs

Added IfSelected to FilteredSiteMapNodeVisibilityProvider.cs
Added filtering support for the "IfSelected" keyword to FilteredSiteMapNodeVisibilityProvider. Now supporting HTML helper name + "IfSelected" (example: "MenuHelperIfSelected"), named HTML helper + "IfSelected" (example: "MainMenuIfSelected"), and to match every control "IfSelected".