feat(popover, tooltip): add shown state api

[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:

Adds nbPopoverShowStateChange output to the popover directive.
Also adds ability to use popover as a template variable:

<button nbPopover="..." #popover="nbPopover">...</button>

Is shown: {{ popover.isShown }}

Private popover directive properties changed to protected to simplify extension.

Add input for popover css class.

All above applies to tooltip as well.

[codecov]

Codecov Report

Merging #1998 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1998     +/-   ##
=========================================
+ Coverage   84.13%   84.23%   +0.1%     
=========================================
  Files         235      235             
  Lines        7835     7887     +52     
  Branches      684      689      +5     
=========================================
+ Hits         6592     6644     +52     
  Misses       1042     1042             
  Partials      201      201

Impacted Files	Coverage Δ
...nts/cdk/overlay/dynamic/dynamic-overlay-handler.ts	100% <100%> (ø)	⬆️
...work/theme/components/tooltip/tooltip.directive.ts	100% <100%> (ø)	⬆️
...work/theme/components/popover/popover.directive.ts	100% <100%> (ø)	⬆️
.../components/cdk/overlay/dynamic/dynamic-overlay.ts	99.11% <100%> (+0.13%)	⬆️