Add tabs to the left and right hand sides so that we can select both a mode and a view

As in legacy OpenCOR, we want to have tabs on the left hand side so that we can select a mode (`Editing` or `Simulation` mode, at this stage) and other tabs on the right hand side so that we can select either an editing view (e.g., the `CellDL` view) or a simulation view (e.g., the `Simulation Experiment` view).