• add new addGrid(parent, opts) to create a grid and load children instead of init() + load(), which is used by load() to supports nested grids creation.
  see nested.html demo.
• save() will now work on nested grids, recursively saving info. added flag to also allow saving the current grid options + children
  (needed for nested grids) so you can now call new adddGrid() to re-create everything from JSON.
• fix 1505 don't call movable()/resizable() on locked items error. thanks @infime
• fix 1517 force typescript 3.6 as 3.7+ has breaking change forcing apps to be at least 3.7 (we only used one tiny feature in newest TS, not worth the incompatibility).