Add other matplotlib color options.

[TheBrigandier]

Need to update colormap settings select input to include optgroups with the other color types matplotlib supports. Right now there's only support for the general heatmap specific colors, but it may be helpful and accessible to offer the other color types. The optgroups will let the default colors be sorted towards the top, with the more exotic choices in their own groups below.

[draeath]

It would be nice if one could simply enter a color map directly, below the dropdown (eg, the colormap identifier that would be passed to matplotlib). This would allow a user to pick anything their installed matplotlib could use, without further changes to PrusaMeshMap.

Ideally, catch exceptions thrown from invalid entries (absent, invalid, etc) and fall back to the item selected in the dropdown.