* Comment tweak
* Directly print traceback
Since the shell.showtraceback is giving some issues
* Make InteracrtiveSceneEmbed into a class
This way it can keep track of it's internal shell; use of get_ipython has a finicky relationship with reloading.
* Move remaining checkpoint_paste logic into scene_embed.py
This involved making a few context managers for Scene: temp_record, temp_skip, temp_progress_bar, which seem useful in and of themselves.
* Change null key to be the empty string
* Ensure temporary svg paths for Text are deleted
* Remove unused dict_ops.py functions
* Remove break_into_partial_movies from file_writer configuration
* Rewrite guarantee_existence using Path
* Clean up SceneFileWriter
It had a number of vestigial functions no longer used, and some setup that could be made more organized.
* Remove --save_pngs CLI arg (which did nothing)
* Add --subdivide CLI arg
* Remove add_extension_if_not_present
* Remove get_sorted_integer_files
* Have find_file return Path
* Minor clean up
* Clean up num_tex_symbols
* Fix find_file
* Minor cleanup for extract_scene.py
* Add preview_frame_while_skipping option to scene config
* Use shell.showtraceback function
* Move keybindings to config, instead of in-place constants
* Replace DEGREES -> DEG
