Implement setting groups, new PrefsAPI method

You can now create groups of settings, which can be hidden by the user.
Also, there's a new method in the PreferencesAPI: createCheckbox. It allows to create checkboxes easily.
A bit of refactoring in some sections of the code.
New setting: markdown support

Implemented using https://github.com/alfateam123/md2bbc. Thanks @alfateam123!