Fix bug when dragging project to new window

Steps to reproduce:
Focus editor. Drag editor's project to other window. Focus editor again.

Observed:
Error in logs, editor not focused.

Expected:
Editor focused.
Fix broken File > New > Document
Move WindowController to editxt.platform.mac.window
Add "test" platform

This provides mock implementations of platform specific features that are hard to deal with in tests.
Move window main view setup into platform module
Improve focus next/recent editor after (re)move
Refactor Window.insert_items tests

...and current project logic.
Get first uncollapsed project if no current item
Fix bug on project becomes current in window
Improve Window.insert_items docstring