Pygame-menu

- Added image background example.
- Added `set_background_color_opacity` method to Theme class.
- Added `None` to area parameter of `draw` BaseImage method class.

- Added widgets `.set_attribute()` and `.get_attribute()` methods.
- Move Menu `back_box` constructor to `menubar_close_button` Theme, 204.
- Improve pygame error messages.
- Improve title string casting.
- Added Button `.update_callback()` method.
- Fix pygame min version, 206.

- Ignore mouse if selection time is lower than the repeat limit.
- Added selection time to widgets, use `widget.get_selected_time()`.
- Added font antialias to themes.
- Added `widget.set_title()` method.
- Improve compatibility with pygame v2.
- Improve text input history on cut/copy.
- Improve auto separator completion in ColorInput.
- Improve method general assertions.
- Fixed issues 198, 199, 202, 203.

- Fixed assertion error on custom theme, 196.

This version introduces a lot of changes from v2:
- New documentation page: https://pygame-menu.readthedocs.io.
- Scroll support.
- Image support, widget and background.
- Added themes.
- As the library has been rewritten, many changes have to be made from v2 to v3. See migration guide.

Thanks to:
- anxuae
- eforgacs

[Migration guide](https://pygame-menu.readthedocs.io/en/latest/_source/migration_guide.html)

- Clicking outside the selected widget disables active status.