make PdfExportService reusable in debug mode

This refactors the service so that tests can easily access the produced
debug HTML

rector and codesnifffer fixes

Handle styles

We now have a Styles class that takes care of loading the styles from
plugins etc.

first start at refactoring

We now have a few classes that separate the concerns

DokuPDF - inherits from mPDF and doesn't do much anymore except for a
few internal defaults and overwrite

first start at refactoring

We now have a few classes that separate the concerns

DokuPDF - inherits from mPDF and doesn't do much anymore except for a
few internal defaults and overwrites
Config - encapsulate the configuration (file and input vars) and created
an mpdf config array
Template - handles loading the template files and applies placeholders
Writer - composites the above classed and gives access to write
operations

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