api cleanup: use ByTag consistently (emphasises slowness of lookup)

* revert last commit from laurent
* new getNumElements() et getElement() functions in GEdge/Face/Region to
  simplify access to mesh

getTypeOfElements  and addThisTypeOfElement
to GEntities

added
getTypeOfElements  from Gentities and Physical Groups in GModel

API cleanup: removed unused functions

make parser embeddable in code with other parser (yy->gmsh_yy)
more work on headers
fixed lots of gcc warnings

start reducing header pollution (removed Gmsh.h + cleaned up GModel & co)

- fixed missing return value in OCCVertex::reparamOnFace

- added code to highlight orphan entities (dave's idea)

- added gui for incomplete second order elements

- removed ugly static variables in 1d algo

- made sure we remove all 2d order vertices before generating new ones

fix interactive modification of mesh attributes

Plane surfaces are now recognized and the
usual gmsh "cross" is drawn correctly

prepare for extrusion

add comments

- added 'Delete' button in visibility browser so that we can easily remove
  physicals and partitions from a model

- slightly changed the behaviour of "Save all (discard physicals)" in the msh
  format: now we really discard the physicals, and set the physical-region
  tag to 0 in the msh file. This is the most natural thing to do: this
  way when on reads a msh file, we get the correct information back.