Add checkboxes support

[rychkog]

This is extended version of PR #181 by @Tmaster

[codecov]

Codecov Report

Merging #211 into develop will decrease coverage by 0.84%.
The diff coverage is 88.61%.

@@             Coverage Diff             @@
##           develop     #211      +/-   ##
===========================================
- Coverage    95.86%   95.02%   -0.85%     
===========================================
  Files           23       25       +2     
  Lines          823      965     +142     
  Branches       120      133      +13     
===========================================
+ Hits           789      917     +128     
- Misses          16       24       +8     
- Partials        18       24       +6

Impacted Files	Coverage Δ
test/data-provider/tree.data-provider.ts	100% <ø> (ø)	⬆️
src/tree.service.ts	100% <100%> (ø)	⬆️
src/tree.types.ts	100% <100%> (ø)	⬆️
src/tree-controller.ts	100% <100%> (ø)	⬆️
src/tree.events.ts	91.3% <100%> (+2.11%)	⬆️
src/tree.component.ts	91.22% <100%> (+1.03%)	⬆️
src/rxjs-imports.ts	100% <100%> (ø)	⬆️
src/tree-internal.component.ts	87.93% <80.3%> (-5.35%)	⬇️
src/tree.ts	93.01% <91.66%> (-0.11%)	⬇️
src/tree.module.ts	100% <0%> (ø)
... and 1 more

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