completed basic interface for new fields/fieldgroups/steps/new_submission
this commit is the twin of https://github.com/globaleaks/GLBackend/commit/f8c302ddb4a5e3d85b91e3430296fc61b05268cd
the code of the GLClient will need some improval to prevent some actions that are also
prevented by the backend like associating the same field to multiple fieldgroups or to multiple steps.
i'm still thinking about this things so i've not integrated the relevant code.