Batgrl

TextPanel added for multiline static text.

This release adds a `BezierCurve` type and a coroutine `move_along_path` that will move a gadget along a sequence of curves. `move_along_path` also accepts different easings to control movement. Additional changes include:
- `VideoPlayer` and `BrailleVideoPlayer` renamed `Video` and `BrailleVideo`, respectively.
- `clear()` method added to `Text` and `Graphics` and `shift()` method added to `Text`.
- Gadget minimum height/width is now 0 from 1.
- `on_progress` parameter of `tween()` method of `Gadget` now accepts a callable with a single argument (the argument is the current progress of the tween).
- Text effects added. A recreation of some effects from https://github.com/ChrisBuilds/terminaltexteffects.

Introducing the new canvas data type, Cell! Major changes:
- Char renamed to Cell.
- Cell has additional fields "fg_color" and "bg_color".
- `colors` ndarray of Text and _Root gadgets removed.
- ColorPair removed.

Additional changes:
- Theming is now done with a dictionary of hexcodes.
- GadgetBase is now Gadget.
- Pane gadget replaces the old `Gadget` type. Pane also alpha composites.
- Transparency semantics for text gadgets updated. Can now alpha composite with
the background of transparent text.
- An `alpha` parameter/attribute has been added to several gadgets to use new
compositing.
- `render()` method is now private, renamed `_render()`.
- Resizable behavior removed.
- The following gadgets no longer inherit from Text, but prefer composition:
BoxImage, BrailleImage, BrailleVideoPlayer, ProgressBar, Slider, Sparkline
- Running code in a separate thread is now optional for Console.
- MenuItem is now private, renamed _MenuItem.

- Fixed missing `_pos` attribute from `_Root`. This caused errors when getting `top`, `left`, `right`, `bottom` attributes.
- Tasks returned by videos and animations don't propagate cancellations. Unwanted behavior could occur if while awaiting these tasks the video was paused or stopped.

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- Markdown gadget added/mistletoe added as a dependency.
- `add_text` funcion and `add_str`, and `set_text` methods can now accept batgrl-flavored inline markdown if `markdown=True`.
- Multi-line paste for console gadget.
- Doc strings now conform to pydocstyle.
- Raycaster refactored.
- Three new Text borders added: `near`, `mcgugan_tall`, `mcgugan_wide`.
- Cursor save/restore during rendering re-added.
- VideoPlayer and BrailleVideoPlayer raise FileNotFound if file doesn't exist to prevent cv2 from eating the errors.
- Render lock is now re-entrant. Gadgets must acquire render lock to resize or reposition.