Encapsulate toolkit-dependent global drawing functions in new
drawContextGlobal class, and add a static drawContextGlobal to
drawContext, with a default version that does nothing.

This allows to have a completely virtualized drawing system: we can
now ship a library with a fully working graphic system, indepedent of
the widget toolkit. A client code just needs to subclass
drawContextGlobal to get working graphics (see
e.g. api_demos/mainAntTweakBar.cpp)

- pretty print code

set better default char length (avg of existing line ele sizes) in
addOCCMeshConstraints

never ever change the immutable vertex numbers: otherwise we cannot use
our algorithms from outside with prescribed boundary meshes

-> use MVertex::_index instead

(renamed setNum to forceNum to make sure we don't use setNum by mistake)

enhence statreport

make Show/Hide work with GModel entities

fix crash in remeshUnrecoveredEdges

remove "return" added in orientMeshGFace in revision 1.183

this fixes the surface mesh orientation problem (wrong in 2.4.0)

fix crash in remeshUnrecoveredEdges

fix cl comp pb for embedded verts

rewrote gmsh2DMeshGenerator to make it thread-safe

*** PLEASE TEST THAT THE 2D MESH GENERATORS BEHAVE EXACTLY AS BEFORE ***

fix warnings