Improve estimated output size by querying server

[marikavertzonis]

As per the explanation here
https://github.com/Esri/TilePackageKreator/blob/master/TilePackage/TilePackageEstimateSize.qml
the estimated output size is calculated by the number of pixels divided by tile size. But to get a true estimate the server must be queried.
Instead of continually querying the server - add an option for the user to obtain an improved estimate - so that they have a better idea of the size before pressing the create tile package button.