To support both `ext-workspace-v1` and `cosmic-workspace-unstable-v1`,
the API exposed by `wayland/protocols/workspace` now uses the ext
workspace `State` and `GroupCapabilties` bitfields, and converts them to
the cosmic types for the cosmic implementation.
`WorkspaceCapabilities` is a custom type that has cosmic-specific and
ext-specific variants, and is mapped on both backends.
The ext protocol adds an `.assign` request on workspaces, which is
added here, though not currently used.
It also adds an `.id` event. Which we'll probably want when we have
persistent workspaces, but it isn't needed currently.
We still need to add an extension protocol of ext-workspaces to replace
a couple cosmic protocol features.
