QtSupport: More helper methods for BaseQtVersion

Add a method to query debug/release build availability and one for
the documentation path.

Qbs wants to know those and I do not want to hardcode QtSupport
details there.

Change-Id: I32c83167270dd34a81c4ab3f65e9d92b8f6bca90
Reviewed-by: Daniel Teske <daniel.teske@digia.com>