Added multidrag to slide components.
Added back Shift as toggle key for multiselection of slide components.
Tuned styles of the slide snapshot placeholder to avoid flickering on moving slides. Fixed some formatting issues in code.
Fixed Shift+click multiselect for slides.
Fixed undo on overview mode, fixed multi-select after sorting slides

GH-200
Preserves transition order after sorting slides GH-200
Hanles the case when a slide is added to an empty presentation
Updated silly math to compensate for increased border sizes