Categories display option, small improvements

[my-curiosity]

Hello @andreasschenkel ,

following #9, this PR adds new functionality that we found useful:

1. A top level category can be shown for the courses in table

• this is disabled by default, configurable and can be used with/ instead of/ without duration time ranges depending on users' preference
• moodle/category:viewhiddencategories permission is respected: category names are only displayed when they are visible or when the user is allowed to see them in course context, otherwise get_string('categoryhidden') is returned

2. Set background color of non-standard roles to #eee (equal to guest color) instead of white for better contrast

• the solution is not very clean and uses CSS wildcards, so we could modify or remove this part as needed

3. Added : between role name and amount of courses for readability, for example "Manager : 1 Course"

4. Simplified some expressions in code

5. Set required Moodle version to 4.0

Let me know if any additional changes are needed, for example if you plan to merge the develop branch and it causes conflicts.

[andreasschenkel]

Thank you for your new feature pull request. Please have a look at the develop branch. I merged your code to the develop branch and did some changes (see commit messages).
Does the develop branch still supports your needs?

[my-curiosity]

Changes in the develop branch are very helpful, thank you for improving the code. Closing this PR as all the requested features are already merged there.