Batgrl

- 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.

* Gadget, Text, and Graphics (and a few other gadgets) now inherit from
`GadgetBase`. This removes `background_char` and `background_color_pair`
from many gadgets where these attributes/parameters did nothing.
* `StackLayout` added. Stack layouts position their children horizontally or
vertically and resize them to use all available space. Child height or width
proportions can be set with the `proportions` attribute.
* `add_syntax_highlight` added to text gadgets. `Pygments` added as a dependency.

Additional Changes
- `ColorTheme` is now a dataclass (instead of a `NamedTuple`).
- `ScrollView` is themable (again)
- Character attributes of gadgets, such as `Text.default_char` or
`Gadget.background_char` can now be set with styled `Char`.
- Gadget size and position are locked during rendering for thread safety.
- Gadgets that inherited scroll view now inherit `GadgetBase` and have a
scroll view.
- Updated description of transparency for Gadget.
- Fix several occurrences of improper escapes in strings.
- Moved `Char`, `style_char`, and `coerce_char` into `text_tools`.
- All layouts (except split layout) remove hints from children.

- Console gadget added.
- Select all and select word key-binds added to text input gadgets.
- Method signatures added to method stubs in doc strings.
- Several bug fixes.

- Renamed project to `batgrl`. Badass terminal graphics library. Widgets are now gadgets for flavor.
- Rendering now uses regions by default, but painter's algorithm can still be used by passing the rendering mode to an App.