Use args_from_dict in all camera classes

[NiklasNeugebauer]

• remove overwrite of from_dict in UsbCamera and MjpegCamera
• use args_from_dict in RtspCamera

[codingpaula]

I added some tests for this. What do you think?
If they run successfully, I'd say we're ready to merge.

[NiklasNeugebauer]

I added some tests for this. What do you think?
Good idea.

Some notes:
I think it is quite confusing to test something that is part of Camera inside of a specific test. For example, test_storing_configurable_camera_as_dict is the only one that checks history length and base path are restored. This can be difficult to understand if a test fails and somewhat difficult to maintain.

We do not actually test the functionality that we just fixed. A base CalibratableCamera was always able to load its calibration but the subclasses overwrote its behavior.

I'll look into fixing those two points.

[NiklasNeugebauer]

seems a little overkill in hindsight but that is what I ended up with