Change tab interface to have no home button and make `+` open a splash tab

[adamkewley]

Suggested by @tgeijten

Instead of prompting the user to select an editor, mesh importer, etc. or expecting them to use the home button, have the + button spawn a new splash screen from which the user can see their recently-opened documents etc. and immediately pivot into a new UI.

For reference, chrome + edge + firefox have this behavior:

• When the user opens the app, it shows a single "splash screen" tab
• When the user clicks +, it spawns a new (seperate) "splash screen" tab
• Opening something in the splash screen (e.g. clicking a link) replaces the splash screen with the thing the user wants to do
• Closing all tabs closes the entire application - there is no "no tabs open" UI state. The application always has (1, n] n>=1 tabs open