Add keyboard shortcuts and scale bars

[sezelt]

This PR adds the ability to move the detectors with the keyboard: WASD moves the diffraction detector and IJKL moves the real-space selector.

Additionally, this adds dynamic scale bars to all of the views, which are calibrated in physical units if calibrations are available in the file.

To-dos:

• Decide how to handle FFT scale bar for non-square scans
• Add additional keyboard controls for resizing detectors
• Update README to mention new controls, and perhaps add a help menu.

[sezelt]

This update now adds a variety of new features above what was originally planned.

• Descriptions of the virtual images are moved to a new bottom bar, to keep them in view at all times.
• Autoscaling of the intensities of each image is now directly controllable via buttons in the bottom bar, which allow auto scaling to happen once, always, or never.
• Switching between detectors now applies a single auto scaling in a consistent way, and hopefully no longer causes odd effects when switching back and forth.
• The FFT view can display either the virtual image FFT or the EWPC of the diffraction pattern.

[sezelt]

I am going to merge this to make what should be the final version 0.9x release before moving to version 1.0.0 (and using semantic versioning rather than 9's)