Improve Camera and move it to another package

[SimonDanisch]

Since I want to make GLVisualize independent of OpenGL, we also need to have the Camera be independent of GLAbstraction.
Also it would be desirable to remove as much Signals as possible to lower latency and secondary signals (map(x-> push!(y, x), Signal(...))).
I've create a prototype for a rectangular selection zoom camera, which needs to be integrated with the current default cameras.
Also it would be nice to have different camera control sets, e.g. optimized for text display, which the user can freely switch and configure.