WIP: Explore option for multiselect in QuickOpen picker

[gjsjohnmurray]

This PR was inspired by #66971 (comment)

It is currently WIP. The UX is ready to see and try, but support for actually opening all items in the multiselect case is still a TODO.

Pinging @bpasero

@misolori I'm not sure if I'm following the correct convention for toggle buttons whose icon changes. Mine show the icon representing the state you get after you click it.

I also experimented with check and check-all but as it's a file picker I decided file and files made more sense.

[bpasero]

@TylerLeonhardt is the owner of quick pick for a while now.

[gjsjohnmurray]

@bpasero thanks for the redirect.

As part of this I changed QuickInputController to permit icons in the customButton label.

[miguelsolorio]

Initial impressions: I'm not sure I am a fan of having a button toggle as part of the file picker, I'd be more inclined to introduce a new command that opened a multi-select quick pick instead.

[TylerLeonhardt]

Yeah I have the same thoughts as @misolori. I think we could have quick access provider specifically for multi-select that might be interesting. If you look for QuickAccessProvider in the code, you can see them. Also, if you open the Quick Open and use the prefix ? you can see them all.

I will say, this would be the first QuickAccessProvider that supports multi-select so there might be additional work to light that up so it only shows when you use that provider.

[gjsjohnmurray]

A drawback of doing this as a separate quick access provider (triggered by a '+ ' prefix, for example) is likely to be discoverability. I predict a lot of users won't find it.

[TylerLeonhardt]

Yeah that's always a challenge with new quick access providers... but I think we want to work on that problem of discoverability in issues like #115641

[miguelsolorio]

The other factor to consider is, how many people are asking for this? When only a small percentage of users want something while introducing more noise to the majority is where it makes it harder to accept changes like these. That's why introducing this as a command or "mode" for those that do want it makes more sense, even if it isn't very discoverable (this isn't the only instance).