Highlight pane or elements in the Editor UI

**Is your feature request related to a problem? Please describe (REQUIRED):**
Allow to create a highlight frame over the existing panes in the Editor to allow creating user tutorials with Editor Scripts.

**Describe the solution you'd like (REQUIRED):**
Made it possible to define over which Pane the highlight should be existing. It should be automatically resized with the actual Pane's size.

**Describe alternatives you've considered (REQUIRED):**
Alternatively, if it might be easier to implement - allow to modify brightness of the panel (we already can do it via CSS actually, so maybe it's possible to modify it via Editor Scripts).

**Additional context (OPTIONAL):**

Examples from Unity: