Moves the abstract renderer out of the firectory with the renderer to prohibit warnings from the renderer class finding mechanism.
Removes the require_once from the RendererFactoryRegistry and also removes the global execution code.

Global execution code is a source of problems and since we have found a
clever way to use the class loader for finding our classes we can now remove
this code. Furthermore we added more error checking for plugin classes
and will warn the user about its error.
Adds comments to make build system happy. Additionally some comments are improved.
Adds missing use or i18n.
Extend placeholder class to support custom formatting of the content.
Adds StorageLocation to Placeholders. Now the data of the StorageLocation is accessible again.
Makes ZebraLabelWriterRenderer more general and add rendering of StorageLocations to it.
Cleanup PDFDefaultRenderer and make the methods to render object independent.
Now the renderer is flexible enough to use the configuration to describe the
label for many common cases.
Additionally adds 1D barcodes to be used with parts too.
Include PDF rendering stuff only if the correct configuration is set.
This will keep PartKeepr running also, if TCPDF is not installed. PDF
printing is then not available.

fixes #283