Add bodygraph with wetness data

[Kromgart]

Summary

Features "Add bodygraph displaying wetness status"

Purpose of change

I was making a custom sidebar and noticed that there are bodygraphs for hitpoints, encumbrance, body temperature and bodypart status, but wetness was missing. So I decided to add it to expand modding options.

Describe the solution

This PR adds another bodygraph, which displays wetness of the corresponding body part. The bodygraph can be used same as the other 4 to build custom sidebar widgets (see the attached screenshot).

Sample widget json:

  {
    "id": "body_graph_wet",
    "type": "widget",
    "copy-from": "compact_body_graph_template",
    "var": "body_graph_wet"
  }

Wetness is color-coded as follows:

0%               - light gray (Dry)
0%  <  ... < 30% - light cyan (Damp)
30% <= ... < 60% - light blue (Wet)
60% <= ...       - blue (Soaked)

I took these breakpoints from the code that applies the wetness effect (src/character_body.cpp), and moved them to 2 new constants in weather.h

Describe alternatives you've considered

Testing

Created a widget in a custom sidebar (sample code above), walked around, got wet, got dry (slowly), observed that the bodygraph colors match wetness effects in player info screen '@' and body part wetness percentage in Body Status screen '{'.

Additional context