add NbIcon on NbWindow title

[Salvatore-Als]

Please read and mark the following check list before creating a pull request:

• [V] I read and followed the CONTRIBUTING.md guide.
• [V] I read and followed the New Feature Checklist guide.

Short description of what this resolves:

This feature allows to add icons in the title of a window.
I made this addition for a personal need, asked few month ago #2576

[codecov]

Codecov Report

Merging #2686 (d21a4f6) into master (db9e556) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head d21a4f6 differs from pull request most recent head 3ce7ae6. Consider uploading reports for the commit 3ce7ae6 to get more accurate results

@@            Coverage Diff             @@
##           master    #2686      +/-   ##
==========================================
- Coverage   77.82%   77.78%   -0.04%     
==========================================
  Files         262      262              
  Lines        8229     8228       -1     
  Branches      896      896              
==========================================
- Hits         6404     6400       -4     
- Misses       1550     1553       +3     
  Partials      275      275              

Impacted Files	Coverage Δ
...mework/theme/components/window/window.component.ts	82.35% <ø> (ø)
...ramework/theme/components/window/window.options.ts	100.00% <100.00%> (ø)
...rk/theme/components/radio/radio-group.component.ts	91.01% <0.00%> (-3.50%)	⬇️

[Salvatore-Als]

UP.
Can we have a review for this feature ?

I think it would be a good idea to add it natively to nebular :)

[yggg]

Closed by #2915