Make :debug-webaction public

[The-Compiler]

For some web actions exposed by Qt, it actually makes sense to let the user invoke them. Right now I'm mainly thinking of:

• Copy (see the discussion in Yank as rich format #5467 (comment))
• Paste (see cannot paste content in cryptpad #6029)

But perhaps there are more.

For copy it might be possible to integrate it into :yank, but for paste we don't have a matching command yet. Maybe it'd make sense to have that and try to fit the actions into existing commands (say, :undo --edit if we decide to expose that) - but it seems like that might get hairy.

Instead, we could have a :webaction with a couple of supported actions, and if someone wants to run one by name, they could still use :webaction --unsupported Undo to do what :debug-webaction does now.

Perhaps it'd even make sense to switch from :fullscreen --leave to :webaction exit-fullscreen, not sure.