Editor Help dialog shortcut on different platforms and the iframe title

[afercia]

When entering the Editor, screen readers announce the title attribute set on the Editor iframe. Iframes must always have a title attribute to give context to users. By the way, the current title text is always the same, regardless of the platform:

Rich Text Area. Press Alt-Shift-H for help

While Alt-Shift-H perfectly works on Windows, on a Mac the keyboard shortcut is different and it's Control-Alt-H. Mac and VoiceOver users will still ear "Alt-Shift-H" though.

It would be great to use a different iframe title attribute depending on the used platform. Maybe also changing it on the fly with JavaScript could work (would need some testing).

[afercia]

36863.diff​ is a first pass, uses tinymce.Env.mac which is the same check used the keyboard shortcuts help dialog. Screenshot on Safari+VoiceOver:

[azaozz]

Patch looks good, lets fix this :)

There is already preInit callback in there, no need for another. Also there is editor.iframeElement now.

[azaozz]

In 38110:

TinyMCE: replace the editor iframe title on MacOS to fix the help shortcut.

Props afercia, azaozz.
Fixes #36863.