Add XZ grid in EditView

[marques-bruno]

• Fixed grid on XZ plane, with resizable units (in EditView's toolPanel)
  In a scene in meters, a cube of size 1 will take 1 square, in mm it will take
• XYZ Axis displayed as lines in the EditView to replace the frame

Still todo before merge:

• Change grid size depending on zfar instead of arbitrary 1000units
• adapt texture resolution according to grid subdivision (or explore mipmapping?)
• remove previous frame view and toggle XYZ axis using previous frame checkbox in toolpanel

[marques-bruno]

It works. Not perfect, but will do for now