fix: regression onSelectedRowsChanged not receiving correct caller

[ghiscoding]

• a regression was introduced when dropping jQuery, the SlickEvent structure changed in the notify function. Previously a SlickEvent would accept a CustomEvent directly and the previous code was expecting that event to exists and override its CustomEvent detail, however the newer approach is to always use a SlickEventData and no longer use the CustomEvent directly and this caused the regression since the SlickEventData doesn't have a detail property but rather something like this SlickEventData { event: { detail } }
• the fix is to simply create a CustomEvent with { detail: caller } which we then pass to the SlickEventData constructor so that our caller isn't lost and rather reused later when triggered by onSelectedRowsChanged

below is where it changed in SlickGrid slick.core.js file (we can see on the right side the new behavior is to always convert to a SlickEventData and that caused the regression)