Keep project groups together in the new sidebar and let me hide projects

[Xyttra]

In the new-style sidebar, sessions move to the bottom when they’re active. Could this behavior work with the existing group by project view, so that when any session in a project is active, the entire project group moves to the bottom together?

More importantly, the ability to toggle visibility for certain projects on or off. I may not want to settle a specific project but also not work on it yet; this would prevent a cluttered sidebar.

[chomamateusz]

For the second half of this (turning projects off without settling them): I have been running a Hide mode on the project filter for a few weeks. The same selection can mean "show only these" or "show everything except these", and each mode keeps its own saved list, so hiding a few noisy projects does not cost me my working set. Screenshots: #36370