Force dropdown opening downwards when intersecting viewport top

[duredhel13]

trafficstars

Summary

Replace me with a description of changes. Include screenshots where appropriate.

Checklist

• [ ] *Added unit tests
• [x] *Added a code reviewer
• [x] Added changes to /projects/swimlane/ngx-ui/CHANGELOG.md under HEAD (Unreleased)
• [ ] Updated the demo page
• [ ] Included screenshots of visual changes

*required

[Hypercubed]

Code LGTM... I'll test tomarrow.