• fix #149 #1094 #1605 #1534 re-write of the collision code - fixing 6 years old most requested request

1. you can now swap items of the same size (vertical/horizontal) when grid is full, and is the default in float:false (top gravity) as it feels more natural. Could add Alt key for swap vs push behavior later.
2. Dragging up and down now behave the same (used to require push WAY down past to swap/append). Also much more efficient collision code.
3. handle mid point of dragged over items (>50%) rather than just a new row/column and check for the most covered item when multiple collide.

• fix #393 #1612 #1578 re-write of the drag in/out code - fixing 5 years old bug

1. we now remove item when cursor leaves (acceptWidgets case using dropout event) or shape is outside (re-using same method) and re-insert on cursor enter (since we only get dropover event). Should not be possible to have 2 placeholders which confuses the grids.
2. major re-write and cleanup of the drag in/out. Vars have been renamed and fully documented as I couldn't understand the legacy buggy code.
3. removed any over trash delay feedback as I don't see the point and could introduce race conditions.

• fix 1617 FireFox DOM order issue. Thanks @marcel-necker
• fix changing column # column(n) now resizes cellHeight:'auto' to keep square
• add 1616 drag | resize events while dragging. Thanks @MrCorba
• add 1637 GridStack.setupDragIn() so user can update external draggable after the grid has been created