Add an explorer/title menu contribution point

[eamodio]

Currently extensions can't contribute to the Explorer's title menu

It would be great to allow this. For example it would allow for a better UX in the Toggle Excluded Files to add the toggle there rather than omni-present in the statusbar (See #103536 for an ask)

[isidorn]

@eamodio I think this is a fair request.
However, I would be worried that too many actions end up being there which would lead to too many details.

So for now no immediate plans but let's leave in the backlog and I am open to allow this in the future if there are more good use cases.

[eamodio]

😢

[gjsjohnmurray]

However, I would be worried that too many actions end up being there which would lead to too many details.

@isidorn that could be mitigated by implementing a '...' secondary actions menubutton which would appear if at least one contribution to explorer/title specified a group that disqualifies it from being a button on the title. The `scm/title' menu operates this way.