Load main scene if defined when running editor from the command line
Created by: akien-mga
If no main scene is defined, or if it's an empty string, the previous behaviour will still be used, i.e. launch the project manager. A small fix was also added not to maximize the window when opening the project manager via -editor (i.e. "godot" and "godot -e" will behave the same when no project is in the path).
Fixes #2869 (closed).