Allow viewing up to three panels side by side in horizontal layout

[stevencl]

If we support horizontal layouts we should mimic the mechanism we use in the editor to view multiple panels side by side. For example, if we start out with a horizontal panel for inspectors when in debug mode like so:

We could then support an 'Open to the side' action that would allow the user to open two or more panels side by side:

This would then show the panel alongside the main panel:

Panels would expose a close button which when clicked would regroup that panel with the main panel:

[stevencl]

After a few design iterations and discussions with the team we don't think that this is the right thing to do just now. We might revisit this once the horizontal panel is ready but I will close this until that point.