fix(tabset): wrap lazy load content into template

[yggg]

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

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

Short description of what this resolves:

TODO:

• update docs examples and text
• add/update tests on lazy loading
• probably we need to cache lazy content once it's initialized
• refactoring:
  • extract components into separate files
  • remove <input-name>Value public properties

BREAKING CHANGE:
Following NbTabComponent properties were removed:

• init - use active instead
• lazyLoad - instead wrap lazy content into nb-template with nbTabLazyContent directive.

Closes #1827, closes #1587.

[codecov]

Codecov Report

Merging #1896 into master will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #1896      +/-   ##
==========================================
- Coverage   83.88%   83.88%   -0.01%     
==========================================
  Files         232      232              
  Lines        7595     7594       -1     
  Branches      670      669       -1     
==========================================
- Hits         6371     6370       -1     
  Misses       1027     1027              
  Partials      197      197

Impacted Files	Coverage Δ
...framework/theme/components/tabset/tabset.module.ts	100% <100%> (ø)	⬆️
...mework/theme/components/tabset/tabset.component.ts	91.83% <75%> (-0.17%)	⬇️

[dmk1111]

@yggg any updates on this?