Initial attempt at popups - doesn't quite work yet...
Merge branch 'master' into hints

Conflicts:
	stack/cas/castext.class.php

Added a sub of a hints class.
Updated the hint system to use the syntax [[hint:tag]]
Changes to enable older questions to work.  Still need to fix the editing side, so when a question is loaded the old <hint> is converted to a new [[hint:...]
Change the design so that old <hint> tags are quietly supported.  I still need to make sure these are converted when we load a question to edit it.
Update the edit_form to convert old <hint> to the new style [[hint:..]]
Add a missing language string for the healthcheck.
Fix tidy question when you rename the 1st node in a PRT.
Refactor test setup code, ready for the Behat tests.
Behat tests covering most of the basic STACK features.
Fix codechecker issues.
Further tests.
Minor update to the docs.
Merge branch 'master' of github.com:maths/moodle-qtype_stack
Feature request
Put Maxima code in place to insert *s into variable names.
Fix unit tests
Change the data type of "insert stars" from a Boolean to an integer.  This is a MAJOR CHANGE affecting many parts of the code. It includes an important bug fix around line 243 of castext.class.php where two boolean variables were interchanged.  Having different datatypes here threw an exception, which is helpful.
Feature to assume all variable names typed in my the student are single characters has been added and tested, including addition of unit tests.  Importing questions does not work.
Fix import.