functions and implementation for EditmeshDerivedMesh
- switch drawobject to drawing normals/centers through the DerivedMesh
- added G_DRAW_VNORMALS flag and button, implementation is not yet complete
because editmesh normals are not updated regularly
- switch editmesh draw buttons to use uiDefButBit (can't we get some
monkey to convert all of the uiDefBut calls with TOG|BIT type? It
makes grepping the source much nicer)