Proposal for #2705, display a hint if scrollbar is hidden in placeholder sub menu dropdown
follow guidelines and fix animation behaviours
Merge branch 'develop' of github.com:divio/django-cms into fixes/Issue2705-hidden-scroll

Conflicts:
	cms/static/cms/css/cms.base.css
Merge pull request #2780 from FinalAngel/fixes/Issue2705-hidden-scroll

Fixes/issue2705 hidden scroll