main: Introduce global workspaces adjustment

main: Introduce global workspaces adjustment

And a way to derive "sub"-adjustments from the main one.

The main and private StAdjustment properly represents
workspaces, and has all relevant properties (lower, upper,
and value) set by the workspace manager.

The main adjustment is not bound to any particular actor,
which means we cannot call the 'ease' method directly.

Consumers of this API should create adjustments using
Main.createWorkspacesAdjustment(), and this new adjustment
is bound to the actor that the consumer passed. Consumers
must not change any property of the derived adjustment other
than the 'value' property.

The 'value' property is synchronized between all adjustments
created, which guarantees that all elements that represent
workspaces can have a shared and up-to-date understanding of
where in the workspace layout we are.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2881>