feat(api): add toolbarVisibilityChanged event to the IFrame API

[vishal2005025]

Description

This PR adds a new IFrame API event, toolbarVisibilityChanged, which fires whenever the in-page Jitsi Meet toolbar (toolbox) visibility changes. This allows IFrame API consumers (e.g. apps embedding Jitsi with a custom native toolbar) to react to show/hide transitions.

Fixes : #16469

Implementation

• When the toolbox visibility changes in the web app, the UI logic calls APP.API.notifyToolbarVisibilityChanged(visible) (if APP.API is available).
• In modules/API/API.js, a new notifier method notifyToolbarVisibilityChanged(visible) is added, which forwards:

  { name: 'toolbarVisibilityChanged', visible }

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[vishal2005025]

@saghul @damencho please review and let me know if any changes are required.

[vishal2005025]

@saghul @damencho Please take a look!

[vishal2005025]

@saghul @damencho should i make another pr for this with different branch ?