gradients: Don't crash gammaray

[peter-ha]

gammaray etc. reads the properties and when displaying the value calls the corresponding READ methods. This will lead to crashes when calling "wrong" methods for gradients.

[uwerat]

I'm not 100% convinced by this solution as gammaray shows a wrong information then ( of course this is better than running into an assertion ). Only one of the directions is valid and gammary should display this one only.

Some random ideas - not sure about them:

• maybe QML needs the WRITE method only and we could go without declaring the getters as READ method.
• maybe we can define the property in a way, that gammaray skips it
• maybe there is a way to tell gammary to check the type() first.
• maybe it is possible to replace the 3 properties by 1 property "direction", that is a QVariant.
• ...

[uwerat]

assertions have been weakened to warnings,