tutorial/ -> tutorials/

  Physical Point|Line|Surface|Volume ( "char" , expr ) = ...

allows to specify both a string label and a physical id by hand.

- work on plugin user-friendliness (short help & co)

pp

refactored Opengl_Window.handle()

should behave exactly like the old version, but now it is at least (somewhat)
readable and ready to be generalized for more complicated projection matrices

by convention, for 2d strings, assume that a coord value > 99999 means the
center of the window

add title

more small fixes

Relecture, suite et fin