[structure view] allow users to select the groups to display #1629
Description
By default, the structure view show all groups that are defined via the stereotypes metadata (see #1628). In addition to that, the UI should allow the user to select/deselect individual groups to include or exclude them from the structure tree. This should be a simple check/uncheck option for each group, individual for each project, so this needs to be an action on the project level, not the view level.
The information about the groups that are available for a specific project should come from the language server via a separate command/message. The command to create the project tree content then needs to include the information which groups are selected on the UI to render the corresponding grouping into the tree.