#351 change help text for example preview
peer review comments
#526 replace double quoted strings with single quoted ones

To maintain/improve compatibility with moodle coding standards
we replaced every possible double quoted string through
it's single quoted equivalent.
#524 remove unused variables and methods/functions
#529 move checkmark grading form to a separate file
#530 use symbol's codes instead of symbols themselves for (un)checked examples
#525 avoid some smaller repeating code segments
#524 adapt code to moodle standards (and --> &&, globals on top)
#250 redesign DB-structure and connected methods/functions