Add workspace vars to support DRY when customizing

Add workspace vars to support DRY when customizing

Relabeling the workspaces required the user to update the labels at least in two places of the i3 config file (switch to workspace + move focused container to workspace) to keep a consistent state.

This got even worse when the user started to reference the workspaces elsewhere, e.g. for assigning containers to specific workspaces.

This change supports the DRY principle for users. Relabeling the workspaces is now merely a matter of changing the respective variable ($ws1, $ws2, … $ws10).