tutorial/ -> tutorials/

initializing the api now sets General.Terminal=1 (to print messages by default) and General.AbortOnError=2 (to throw an exception as soon as an error happens) - cf. #1043

- new fltk/isAvailable to detect if GUI has been closed

allow custom onelab button behavior + show how this can be used to code a simple, interactive, multi-threaded python code

- new gmsh::fltk::update function to force an update of the fltk widgets