Delete the following methods from WorkspaceSvg:

- renameVariableById
- deleteVariableById
- createVariable

Modify the following methods on VariableMap to call
this.workspace.refreshToolboxSelection() if this.workspace
is a WorkspaceSvg, replicating the behaviour of the
aforementioned deleted methods and additionally ensuring
that that method is called following any change to the
variable map:

- renameVariable
- changeVariableType
- renameVariableAndUses
- createVariable
- addVariable
- deleteVariable

BREAKING CHANGE:

This change ensures that the toolbox will be refreshed regardless
of what route the VaribleMap was updated, rather than only being
refreshed when it is updated via calls to methods on WorkspaceSvg.

Overall this is much more likely to fix a bug (where the toolbox
wasn't being refreshed when it should have been) than cause one
(by refreshing the toolbox when it shouldn't be), but this is
still a behaviour change which could _conceivably_ result an
unexpected regression.

Also refactor to use named imports core/variables.ts methods.