[WIP] Better invokeMethod #303
                
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This PR is a follow-up of #302. This PR introduces breaking changes and not to ready to be merged (yet).
Here I want to discuss and implement changes to make
invoke_method(and maybe things around it) better.At first, I've changed
invoke_method's return value toOption<QVariant>- which is close to original QMetaObject API and makes it possible to check wether error occurred (this was discussed earlier).@ogoffart, can you give me advice -- is this change OK as it done now? What should be done next?