Builder - Improve drag and drop on row level

[rogerhendriks]

1. Make it possible to drag a field directly from the list to the left or right of an existing field and split the size of both in 2 if the existing one was full width. If not the new field gets the available width.
2. Enable an resize option between 2 fields on the same row so you can drag one larger making the other one smaller at the same time.

[adamberecz]

Released in 1.5.0