Support for the Session class, and add WebForm.Current property to allow to...

Support for the Session class, and add WebForm.Current property to allow to change the application main form.

[GB.WEB.FORM2]
* NEW: DateBox: Open calendar popup on mouse down.
* NEW: Override the Session class, so that its methods that do not have any sense with 'gb.web.form2' do nothing anymore.
* NEW: WebForm.Current is a new property that allows to define the main form of the application that fills its HTML page.
* NEW: Session.Dir is a new property that returns the session directory. It is now '/tmp/gambas.<userid>/session/gb.web.form2'.
* NEW: Session.Timeout is taken into account by session processes.
* BUG: Window titles do not wrap anymore.