support incremental appx generation
For large products, like games, with content directories tens of gigabytes in size, running tools like MakeAppx.exe to build an appx package can take a very long time.
This makes testing the product it its final form (as a real appx, with associated sandbox restrictions, not as a dev-registered folder) a painfully slow process.
This pain could be reduced by supporting add/update/remove file options to MakeAppx. This would allow callers to build an appx once and then make quick incremental changes for only those files that have changed.