Functions plotter

   When accessing to this option, the Enter window appears.

   - The 'Name' zone is used to name the incoming window,
   (if no name is given, function name will be used).

   - The 'Variables' zone is used to define variables.

   - The 'Function' zone is used to enter or define a function.

   - The 'Precision' slider allow you to set the precision :-) of the plotting.

   N.B.: to define a new function, it's unecessary to enter the key-word 'Def'.
   Of course, to plot an existing function (and only an existing one),
   it's unecessary to redefine it, you can just enter it's name
   (without parenthesis and without parameters if you want).

   When your inputs validated, a new plotter window will open.
   This one has a new specific plotter menu.

   Features of the plotter window:

   - You can move one or the two axes with mouse clic and move on them.

   - You can zoom in a range with mouse.
   The zoom window will then appears to modify your inputs.

   - Arrows keys are used to move in the plotter window.