Cleaned up new constraint line drawing while grab/rot/scale

- uses callback mechanism to tell main drawing routine what to do
- for that reason it doesn't use frontbuffer drawing anymore
  and it shows up in all 3d windows as well
- it uses the same colors as for the grid axes (I tweaked it a bit,
  this is based at themecolors, and also should work in different
  background and grid color)
- I disabled drawing lines through every object or every vertex.
  The current display method is clear and not distracting
- when in 'local' transform (double press X/Y/Z), it displays a nice
  axis in the center of transform for vertices.
  In object-mode, local transform differs per object, so constraint lines
  and axes are drawn for each individually...

Also:

- fixed an old bug in rotate transform(). Using a constraint for
  rotation (X, Y, Z) didn't work for multiple objects at all!

source/blender/src/drawobject.c

@@ -269,40 +269,6 @@ void helpline(float *vec)
 	glDrawBuffer(GL_BACK);
 }
 
-void constline(float *center, float *dir, int col)
-{
-	float v1[3], v2[3];
-	short val1[2], val2[2];
-
-
-	VecCopyf(v1, center);
-	VecCopyf(v2, dir);
-	if(G.obedit){
-		Mat4Mul3Vecfl(G.obedit->obmat, v1);
-		VecAddf(v1, v1, G.obedit->obmat[3]);
-	}
-	if(G.obedit) Mat4Mul3Vecfl(G.obedit->obmat, v2);
-
-	project_short_infiniteline(v1, v2, val1, val2);
-
-	persp(0);
-	
-	glDrawBuffer(GL_FRONT);
-	
-	cpack(col);
-
-	setlinestyle(0);
-	glBegin(GL_LINE_STRIP); 
-		glVertex2sv(val1); 
-		glVertex2sv(val2); 
-	glEnd();
-	
-	persp(PERSP_VIEW);
-
-	glFinish(); // flush display for frontbuffer
-	glDrawBuffer(GL_BACK);
-}
-
 void drawaxes(float size)
 {
 	int axis;