Create a custom widget to include controls

[keewis]

This widget is an alternative to #197, but does not support the player nor does it use H3HexagonLayer for h3 cells. The main advantage of having a single widget is that it will make #196 much easier.

This PR also splits up the plotting module into multiple files, which may make working on the code a bit easier.

[relativityhd]

Looks promising! A few features I would miss as a user from #197 :

• "Robust" colorscaling via 2nd and 98th percentile (This is actually very important to me, also to having it enabled by default)
• Per-Variable colorize settings
• Type hints for the explore method of the accessor