Add ability to Insert Element ScriptType in Array
At the moment we can delete elements but not add a new one in the middle of the array.
This is extremely time consuming to add new elements in the middle of the array
@jiajasper Design pass?
Added example project here: https://playcanvas.com/editor/scene/1406644
Please see design for #752 and #753

Changes are as follow:
- Added dragging
- Added a + icon to add new element to the end of the list, then the user can drag it to insert it anywhere within the array
- For some visual hierarchy, we only enable the input box on Array Size when user hover over it
https://www.figma.com/file/kMiAhzD5xlSobeBuU6lxRy/Jasper-PlayCanvas-Hub?node-id=799%3A37321
This feature is very sorely needed on our part! We have a large array of configured JSON attributes and have no way to reorder things at the moment.
Please implement ASAP! Thanks!