A11y Instructions dialog for MAC should mention Cmd+L as well as Control+L shortcuts

[jswiderski]

Type of report

Bug

Provide detailed reproduction steps (if any)

1. Open Safai on Mac, focus editor and press Alt+0 to open A11y dialog.
2. You can see that dialog say Cmd+L is the shortcut for link.
3. Focus dditor and press Cmd+L

Expected result

Link dialog gets opend.

Actual result

What works on Chrome or Firefox, doesn't on Safari as Safari by default uses this shortcut to Open Location (enter different URL):

Other details

The Control+L shortcut works fine in all browsers while Cmd+L works on Firefox, Chrome but not Safari. Taking that into account, the A11y dialog should mention both shortcuts for MAC

• Browser: Safari
• OS: Mac
• CKEditor version: 4.0+
• Installed CKEditor plugins: N/A

[jacekbogdanski]

Line responsible for detecting os functional key: https://github.com/ckeditor/ckeditor4/blob/4ec334ebc8ed38a935406b87044c1c1b0788eaa4/plugins/a11yhelp/dialogs/a11yhelp.js#L82