Draft: Resolve "Allow the additive mode as it is now possible for geomapfish"

Description

closes #256

Job done

• config.json + src/tools/configuration/girafeconfig.ts
  • variable "selectionMode" added under "themes". Values "add", "replace".
• src/components/treeviews.ts
  • function "onThemeChanged" removed
• src/tools/themesmanager.ts
  • changes in function "onChangeTheme"
    • existing layers are kept if config.themes.selectionMode is "add"
  • function "addLayerToLoadedList" deleted

Definition of Done

For the Developer:

• Code compiles and conforms to defined coding standards (eslint, tsc, prettier).
• If necessary (at the developer's discretion), unit tests have been added for critical parts.
• All unittests run without error.
• Sonar-Scan returns no new issues or security hotspots.
• Documentation has been updated where necessary (see documentation repository).
• Merge-Request contains a few explanations of what was done.
• A Reviewer has been assigned to the Merge-Request.

For the Reviewer:

• New code was reviewed.
• If comments could be added on unclear code, the developer has been informed.
• If unitests could be added in certain places, the developer has been informed.
• Merge-Request was merged on main branch.
• All pipelines work.
• The changes were tested on demo environment.
• Application performance has not been degraded by the modifications.

Related to #256