Quick Ghostview Generator design explanation
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libgs has a limitation that there can only be a gs instance per process.

To overcome that problem there is zero (when no gs document has been opened)
or one GSInterpreterCMD threads running (when on or more gs documents are open
in the same okular process)

GSHandler is the libgs dependant code, it could be in GSInterpreterCMD as there
is the same amount of GSHandler and GSInterpreterCMD instances but i thought it 
made sense to separate them

As there is only one GSInterpreterCMD for potentially N GSGenerator, the imageDone
signal from GSInterpreterCMD also emits the request and the GSGenerator checks
if it is its request that was done or from another GSGenerator.