Fixes FilterExtractor to work as expected.
Rewrote UserManager / UserService to use the AbstractManager.
Fix an erroneous comment.
Fix printing dialog and its layout.
Adds a lastSeen field to the user, which is always updated if a service will be called for a logged in user.

This field will later be used to determine if a special client like a printer
service is online or not.
We can now create PrintingJobs if we want to print something.
Such a job is supposed to be executed by a special service which enables
us to forward the file to a printing driver, a special device or something else.
Creates a new simple renderer for Zebra printers. The renderer will create
raw ZPL code which can be sent direclty to a Zebra printer.
Show only online users in target drop down box for printing.
Open the rendering plugin for passing an arbitrary amount of object data.
This also includes, the plugin can now request a desired configuration item
by itself. The frontend will handle this by showing it only if it is needed.
Store the last selected printing configuration and target to the users preferences.
Refactoring: Move some code to a dedicated static method which makes reuse easier.
Move decoding of configuration data for the Renderers to the renderer itself.

This makes an interpretation of the configuration data more flexible.
Now the renderer can use its own format if needed. The json format should
be preferred, but may be bad if too much escaping must be added.
Move placeholder replacement and which ifo is replaced to a dedicated class for reuse.
Move PErcentOrNumericHelper to Utils directory.