question bank: MDL-12719, MDL-15774, MDL-17105, and MDL-18174

question bank: MDL-12719, MDL-15774, MDL-17105, and MDL-18174

These are all to do with the combination of the $form->category, and $form->categorymoveto fields on the question editing forms since Moodle 1.9. These are now resolved to a single ->category value in question.php, which simplifies the logic for the individual question types.

MDL-15774 and MDL-12719 errors editing Cloze question
MDL-17105 problems with qustion saving logic.
MDL-18174 'Select from subcategories' option not preserved when editing a random question.