Added a mchanism to bring in xmls from optional modules. In module.cmake files,
modules can set global properties named ${vtk-module}_SERVERMANAGER_XMLS and
${vtk-module}_PARAVIEW_GUI_XMLS to servermanager and gui xmls respectively.
SMApplication and Applications/ParaView both look at these properties for all
enabled modules to see if any modules provide xmls and then bring those in.

VisItBridge has been updated to use this mechanism. This fixes the issue with
VisItBridge xmls always being included even with VisItBridge was not enabled
causing dashboard issues.