[Sources] Improve design of thread group headers

[fvsch]

It looks like in some situations we are grouping sources under "Main thread", "Workers", etc.

The UI for those group headers looks a bit raw:

• It adds a bit too much space at the top (we would like to keep items starting at 4px from the top edge).
• Not sure those headers have the same height as other items.
• Horizontal text alignment is a bit off.
• Most importantly, they don't come off as headers. Maybe we should make them bold or find some other way to make sure the grouping is obvious to users.

@jasonLaster do you know:

• if we have an existing UI spec for that, or we should just make it up and try to be consistent with other work on Debugger panes? (Fixing vertical (and horizontal) rhythm in debugger right sidebar ux#45)
• if there are specific steps to reproduce to show those headers? (I'm only seeing them in Nightly, not in launchpad, and only sometimes)

[fvsch]

Current style:

Accordion style:

Maybe we should just go and use the accordion component, making each group collapsible?

Minimal style:

[abhiaecs]

Hey!
Is this bug already claimed? Or can you assign it to me?

[fvsch]

We might need to decide on the design first.

@jasonLaster I remember you said on Slack that you wanted to have a single tree with the threads/workers as the first level nodes?

[jasonLaster]

yep - i was planning on a single tree.

[fvsch]

Fixed by Jason in 69 I believe.