Add workspace creation protocol.

[outfoxxed]

This protocol allows workspaces to be created outside of ext-workspace's create-by-name strategy.

Many compositors create workspaces dynamically based on content or focus. This attempts to encompass all common strategies I'm aware of, and leaves the door open for compositor specific workspace creation methods to use most of the protocol.

This is commonly used by bars and workspace switchers.

[outfoxxed]

addressed review

[YaLTeR]

guess further feedback would need actually trying to impl this

[outfoxxed]

I've exposed capabilities at a workspace-group level, so applications can show UI elements based on the expected capabilities of a given group (e.g. insert button).

I could've done this either as an object you create from a workspace group, or an object that sends events for all workspace groups. I've chosen to do the second as it should be less of a pain to implement in clients, but I'm open to changing that.

Given further thought, this won't be very useful without considerably more data, and the most flexible solution that sets expectations properly is to just not do it. Not exposing anything makes client UI harder to handle but should also force clients not to make incorrect assumptions, and instead use targets to probe for capabilities where they plan to create a workspace.