tree nodes.
Each individual node is responsible for introducing the 'canDrop'
function/flag to decide whether the 'Delete/Drop' context menu should be
enabled/disabled.
In commit-id: 26aa5607ad, 'obj.canDrop'
was set to true in the 'pgBrowser.Node' in delete callback, just to make
the server-group droppable, which is wrong, as all the nodes are
getting affected because of this change in a wrong way.
To fix the issue, added the 'canDrop' function in the server-group node.
Apart from them, also added restriction for not allowing to delete the
default server-group.
Also, handled the same restriction at the server end.
2c7a45814c