Adding Indexes on Expressions

I am currently working on the feature #2595.

Earlier using pgAdmin 4, we are able to create indexes by mentioning columns only, but Indexes on Expressions is about creating expressions of the single columns or multiple columns. This will help to retrieve data in a faster way when we use WHERE or ORDER BY clause. 

I have made a dummy UI for that and divided the Definition tab to 2 tabs such as Definition and Columns. The Columns tab will have the Columns/Expressions control and Include Columns control and all other controls that are earlier present in the Definition tab except Columns control and Include Columns control are kept as it is.

Attached the new design, please review it.

Thanks

Anil Sahoo

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