#77 more context sensivity for menus: primitives and mesh Ops

[Dominique B.]

When the selection does not contain any mesh geometry
but only i.e. primitives, the conext menu shouldn't
diplay 'mesh operations' and mesh operations in the
main menu 'Mesh' should be grayed out.

This improves the usability - especially for new users.

Additionally the main menu 'mesh' entries that won't
work in the current component mode, should be grayed
out also.