• fix #1936 some styles left behind after a drag
• remove #1842 incorrect doc/partial code on widget resizeHandles
• doc #2033 resizestop is actually correct as we don't animate resize today
• add - init()|initAll() will now load any listed children (what addGrid() already did)
• add - GridStackOptions.addRemoveCB which is use by frameworks (eg Angular) to dynamically create their gridItem components instead of regular div with class