The protocol states that these should always be sent, but this was
not initially sending bitflags if they were empty. That works, but isn't
what the protocol states.
Not wrapping the bitflag fields in options works well for `Workspace`,
but not for `WorkspaceDataInner`.
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.
