Make color of "mark occurrences" in editor sidebar configurable

### Description

The color of the "mark occurrences" marks in the editor sidebar is not configurable. Depending on color scheme and presence of other marks, the default color isn't very visible. It would be helpful to be able to configure it.

The "mark occurrences" highlighting in the editor contents is configurable under Options > Fonts & Colors > Syntax > All Languages > Mark Occurrence. A secondary highlighting category for the sidebar would be useful, possibly under Syntax > Highlighting.

### Use case/motivation

Accessibility improvemen / personalization.

### Related issues

This is the same issue as in this old Stackoverflow question: https://stackoverflow.com/questions/6207419/color-of-mark-occurrences-in-vertical-bar-in-netbeans

### Are you willing to submit a pull request?

No

Start by tracing the existing Mark Occurrence setting under Options > Fonts & Colors > Syntax > All Languages and the editor sidebar code that renders occurrence marks. Define a separate configurable sidebar highlighting category, then verify that changing it affects sidebar marks without changing editor-content highlighting.

Written by the indexing model from the issue text.