[pgAdmin4][Patch]: RM#1242 - Cleanup busy indication for tree view and tab panes

Hi

*Following are the changes in patch:*

1) Assign a common class '*pg_spinner*' for loading spinner in wcDocker for
all panels.
2) Removed the default wcDocker spinner icon which was specific to page
load.
3) In case of collection node, we were re-creating the grid every time new
data comes. instead it should only reset the collection with new data.
4) Move the code to hide the msgContainer and display grid outside the ajax
which was causing flicker into the dependent panel.
5) Set the timeout for hiding the spinner to 500 inside the success
callback to stop flicker because response is received immediately and cause
flicker.

*Not Fixed:*
Panes should have their content refreshed until and unless the new data
differs from the previous, to reduce flicker.
I am working on this issue.

Please review the patch.

Thanks,
Surinder Kumar