Remove proprietary attribute from data structure

[dialogik]

The proprietary attribute simply means the opposite of the open source attribute. (That is crappy data design; one of both attributes can and should be removed from data structure and from frontend – e.g. list filters!)

(cf. #44)

[dialogik]

After digging into this issue, I found that this has already been discussed (#39) and there is a reason for the existence of both attributes: Open source is defining if there is public access to the source code without any regards to the license whereas proprietary refers to the license of the sourcecode.