Make Matplotlib mode properly usable

[teutoburg]

Problem

The MPL mode is currently a bit of a mess.

Tasks to fix this

To push towards a release, the following things absolutely need to be fixed:

• Cleanup of class in general.
• Reworking the main image generation method.
• Fix the padding issue.
• Make it properly configurabe.

Features to implement

The following features should be implemented to make it properly useable:

• Add scale and compass to images #1
• Choice of using pixel or WCS frame for images.
• Choice of using histogram alongside images.
• Choice of displaying a before-after view from stretching, ideally with multiple stretch modes.

Nice-to-haves

These things could be added but also at a later release:

• Possibility to display multiple sources in one figure, if feasible (low priority).

[Starvexx]

Why not both, WCS and pixel cordinates in the same image?