fix(composite): selected row count always 0 on mass-selected, fix #951

[ghiscoding]

• fixes #951
• add dispose method to container service to start with a blank array whenever we route to another page, this is to avoid reusing already disposed services that might still be in the container service

[codecov]

Codecov Report

Merging #952 (61fb22e) into master (495c6cc) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #952   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          979       979           
  Branches       329       328    -1     
=========================================
  Hits           979       979           

Impacted Files	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <100.00%> (ø)
...es/angular-slickgrid/services/container.service.ts	100.00% <100.00%> (ø)

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