SONAR-5052 Rework widgets drag'n'drop

Add an ability to move a widget below the lowest widget
Revert "SONAR-5052 Drop prototype.js"

This reverts commit 1397f76a963378926a644ef91d623d9c1dec0890.
Revert "SONAR-5052 Drop prototype.js"

This reverts commit 4b989ad947609d9acb0c5715551da2ca22396861.