This PR is a bunch of experiments on my end to improve the performance of the visual editor loading times.
It's been a while since I did these, here is the main points I remember:

• Includes a small fix for temporary characters in visual editor (similar to fix one-off name getting lost in visual editor #2571)
• I made adjustments that allow running the dialogic editor as a scene. This scene has many problems, but it allows using the profiler. This mostly meant putting access to Editor classes/singletons into Engine.is_editor_hint() checks.
• Adds a cache to Editor Settings, improving read performance of those.
• Caches Event icons to prevent constant loading
• Reduces Calls to indent_events()
• Changes the way Extra-Spacing is added to the bottom of the visual timeline editor (because it got recalculated way to much)
• Reduced Calls to update_content_list
• increases the loading batch_size from 10 to 100 because that made things significantly faster (it waits a frame between each batch)
• preloads the common editor fields in the event_block script

There is also an attempt at colored icons in here (for characters) but I scrapped it because unless we have a character color cache this would mean loading all the characters just to get their colors.

Testing and feedback would be very much appreciated.