Create simple mth5 plotter sample

[kkappler]

This issue is about making a simple pre-prototype plotter that just shows five channels from an mth5 file which we can use as an example to embed into the UI.

This can be done with matplotlib, but we later will use the newer tools like panel, holoviews, datashader etc.

[kkappler]

This has been prototyped in Tsvi_Golden.ipynb.

There are a few things to do before we close this out:

The plot button doesn't pop to the eye,

• [x] Make it a different colour and or change type to primary, and or make it less wide
• [x] Select Files button in first window should also primary