bug fixes for the fullscreen mode

[i-just]

Description

Fixes the following issues:

• when using Redactor in a slide-out in the fullscreen mode, the fullscreen editor was hidden behind the slide-out panel
• when using Redactor in the fullscreen mode (in the slide-out and simply on the edit page):
  • the modal was hidden behind the Redactor fullscreen element
  • the dropdowns (for e.g. link, list buttons) were sometimes hidden behind the Redactor fullscreen element

Should be applied to v2 (for Craft 3) and main (for Craft 4).

Related issues

#445