OO-2969: implements a maxima custom operator, add checker for the type of certain variables by import, more code to find the editor and version of the editor in manifest

OO-2969: implements process to mark question as "OpenOLAT" question that the editor can open, implements feedback based on conditions, implements unified feedback editor with condition based feedbacks, make the unknown question panel html match the runtime html markup

OO-1593: fix a critical issue where the content of the QTI 1.2 is not encapsulated in a block element like a p or div

OO-1593: fix some validation errors in qTI 2.1 editor if the user don't click the save button once, add unit tests to check that our item builders generate valid items to start with...