UI Cleanup

[niezbop]

In the perspective of Uplift leaving beta, I believe we should clean its UI. Finalizing the experimental windows, and overall spending some time designing the UI wouldn't go amiss.

What should be targeted in my opinion:

• First and foremost the UpfileEditor.cs, by extracting our custom fields to a common class (something like UpliftGUILayout), as suggested in PackageExport: remove template Upset file #47 , and redesigning this window
• The other windows
• The inspector for the PackageExport

What could be done:

• Maybe create a "hub" with different tabs to have a single GUI entry point?