Refactor config

[3b1b]

Configuration was a bit of a mess, this is an attempt to clean it up and better clarify what's going on.

At a high level, the default_config.yml file should be where all configuration defaults are set, determining default parameters for various classes, constants, or other specifications a user may want to change. They can then create their own custom_config.yml file to update any of these to their own personal taste (for instance mine lives here).

Note, for anyone who had a custom_config.yml file, this update will require that it be updated to a new format

The configuration from these files is loaded into a global dictionary, named manim_config, which is also updated based on the CLI arguments.

Previously, a lot of configuration that used CLI arguments, or these yml files, was a bit spread out an unsystematic. Here the aim is for configuration associated with specific classes to be clearly marked as such, and for code relevant to processing that configuration (e.g. for Window) to lie within the class itself.

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Other updates

• The functionality of ReloadManger is replaced with a simpler run_scenes in main.py, and the updates necessary for the reloading are instead consolidated in reload_scene.
• Bug fix for the use of xelatex

[Splines]

Just as a suggestion, it'd be amazing to squash-merge PRs as it's otherwise a bit difficult to keep track of what changed in Manim on the master branch since we get commits like Change "style" in default config to "text", which are mixed with more "high-level" ones that aggregate multiple changes under one roof, like Refactor config, Add reload() command etc. When browsing through the master branch, it'd be great to just see those high-level commits and if I want to I can still look at the PR and see all the details.