[pgAdmin][RM3269] Slider jumps up issue in DataView panel

The attached patch fixes an issue where slider jumps up when new rows get loaded while scrolling down in the DataView panel in the query tool.

Fix uses rows_affected field to create dummy rows (Apart from the first batch of rows). Dummy rows will be updated as and when they are required. As total required rows are created initially, SlickGrid will not readjust its height when the next batch of rows are loaded. Hence the jump up in the slider will not happen.

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