Improve validation of Qt module.

Now displays all the missing properties, instead of just the first
undefined property in the order we happened to validate them. Also
checks that pluginPath is set for non-static builds (Qt Creator used to
omit this) and various other properties are set and in a valid format.

Change-Id: I96c8a8059b1522c7498b37cba1c40cd4938b10ee
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Docs: fix documentation of cpp.warningLevel

Task-number: QBS-401
Change-Id: Ic7d3c652a1a42501eba1a156a109bb07610d2702
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>