• re-write to native Typescript, removing all JQuery from main code and API (drag&drop plugin still using jqueryui for now)
• add getGridItems() to return list of HTML grid items
• add {dragIn | dragInOptions} grid attributes to handle external drag&drop items
• add save() and load() to serialize grids from JSON, saving all attributes (not just w,h,x,y) #1286
• add margin to replace verticalMargin which affects both dimensions in code, rather than one in code the other in CSS.
  You can now have perfect square cells (default) #723
• fix #1299 many columns round-off error
• fix #1102 loose functionality when they are moved to a new grid
• add optional params to removeWidget() to have quiet mode (no callbacks)
• drop support for IE11 due to more compact ES6 output and newer TS code