Camera image: adjustment of levels + box overlay with dimensions (in um)

[nvladimus]

Hi, everyone, I implemented two convenience features in the Camera Window which I wanted for a long time:

• a button Adjust levels in the bottom left corner to adjust the image intensity between min and max. This allows quick intensity adjustment without fiddling with histogram handles.
• a drop-down list that allows overlaying a Box ROI which displays it's size in um (according to the current zoom). This allows measuring the approximate sample/feature dimensions during the experiment.

I hope you find them as useful as I do!

[ffvoigt]

Thanks for your pull request, I think these are very useful features! I've made a few small changes to keep the demo_config.py defaults and the default colors of the GUI. It might make sense to put the GUI crosshair color into a config option in the future.

[nvladimus]

Thanks for reviewing and merging!