Separate summarizing Group Handlers

To clean up the root summary view, extract the group handlers into their own tab so things like Plugins/Mod Events//etc "summary" views can be seen separately and avoid duplicate views at the depth 0/1 view.

To do this, we will have to start at FST, and walk down the entire tree and mark every handler seen as a descendant.

Then find all handlers not marked and label them a summarizing handler

Then filter summarizing from the TimingsView and show them on a new View.