Upstream now skips the callback, so we need to provide one.

Also, ignore the toggle screenshare shortcut when the video sender limit is reached.

https://google.github.io/mediapipe/solutions/models.html#selfie-segmentation

- add possibility to allow execution of the button's routine besides triggering
`toolbarButtonClicked` API event
- keep backwards compatibility
- get rid of `ToolbarButton`