Eliminate direct access to CfdMeshMgr::m_GridDensity

m_GridDensity is going to move to Vehicle, so this preparation
will make that transition easier.
Move m_GridDensity to Vehicle
Change GridDensity::m_RigorLimit to BoolParm
Remove spaces from GridDensity Parm names.

Encode/DecodeXml don't work with spaces in Parm names.
Read/write CfdGridDensity to VSP file.
Split GridDensity constructor to allow setting useful group name.
Make CFD Grid Density parameters linkable
Add FEA Grid parameters to file and make them linkable.
Convert CfdMeshMgr mesh flags to BoolParm and use getters internally.
Escape key deselects all.

Still need to prevent windows from closing.
Add global handler for ESC key.

For some reason, this doesn't work when the GL window is active,
so the handler in MainGLWindow needs to remain.
Move CFD Mesh settings to stand-alone class.

The CfdMeshSettings class will move to geom-core so it can be
written to file with Vehicle without moving all of CFD Mesh to
geom-core.
Move CfdMeshSettings to stand-alone file in geom-core.
Move CfdMeshSettings from CfdMeshMgr to Vehicle
Wrap GridDensity En/DecodeXml with named node.
Update LinkMgr registry when ParmContainer::ChangeID is called.
Document that CFD file names are not written to file and make protected.
Update CfdMeshSettings parm names to remove whitespace
Add En/DecodeXml wrapper to CfdMeshSettings
Read and write CfdMeshSettings to file.