Detect non-interactive scene start (preview whole scene)

[Splines]

Fixes #84. When starting the scene at a Python "class definition line", we don't start ManimGL in interactive mode (with -se <line_number>). Instead the whole scene is previewed (see the behavior of startStopScene.ts). We now identify this scenario in the executeStartCommand() method and wait not for the first IPython cell to finish, but for the first log info message to be shown:

[17:39:31] INFO                                               scene.py:210
                    Tips: Using the keys `d`, `f`, or `z` you             
                    can interact with the scene. Press                    
                    `command + q` or `esc` to quit 

Known limitations

• After issuing this command, we think that we are in an interactive environment. Therefore, the next Preview Manim Cell will just put checkpoint_paste() in the current terminal and therefore do nothing. But the next Preview Manim Cell will then succeed. If the user exits the shell after having previewed the whole scene, the next Preview Manim Cell will also work fine. I'd like to accept this as known limitation (or maybe if really needed tackle this case in a future PR where we should beforehand discuss what the expected behavior is in this case).

[bhoov]

I can confirm that this fixes #84 -- changes look good to me