Issue #3040850: Add setting polygon fill opacity

Adds a "Fill opacity" setting (0-1, tokens supported). The default 0.3 is the Google Maps default, so existing maps do not change.

This branch is based on the Drupal 11 work in #3433335 (MR !2 (merged)), because the style handling was refactored there. Until !2 (merged) is merged, the diff contains its commits too; the change of this issue is the last commit only.