Currently, if you have an ion-virtual-scroll with a list of items and a search bar for filtering them, when you change the list of items, the items disappear until rendered again, causing a flicker. This could be solved for the items using the itemHeight function to provide the exact height size and bypass some calculations and be more performant etc.
However, if you had a header or footer, they would still flicker. This commit adds two more optional functions named headerHeight and footerHeight that return the exact size of the header and footer respectively and resolve the flicker.
