Ship Qt patch level releases as package updates

The Qt 5 installer right now offers every Qt patch level version as a separate package. This has several drawbacks:

• The number of packages e.g. in the online installer quickly goes out of hand
• E.g. engin.io, cloud packages are only offered for the latest patch level

Instead, the installer should just offer a 'Qt 5.3' package, and patch releases (e.g. 5.3.1, 5.3.2 ...) should be delivered as updates.

(There might be cases though where we managed to break stuff in patch level releases for some people. In this case we should continue keeping offline installers with older patch level releases available).