DuktapeScriptBackend crashes when trying to serialize GLM properties

I'm not sure, whether this is supposed to be an issue of gloperate or cppexpose.

To reproduce: Run gloperate-qt-example and enter `gloperate.canvas0.pipeline.virtualViewport` into script console. The `DuktapeScriptBackend` runs into an infinite recursion, eventually causing a stack overflow:
- During evaluation, `DuktapeObjectWrapper::getPropertyValue` is called on the pipeline object
- The function then retrieves the `virtualViewport` property from the object and converts it to a `Variant`. This variant internally stores a `glm::vec4`.
- Next, `DuktapeScriptBackend::pushToDukStack` is called to return the result to the script via the duktape stack
- Here is the problem: this function operates **only on built-in variant types**, it **does nothing for custom types**, which means the stack is unchanged
- This causes the infinite recursion, as trying to output the result (expected at the top of the stack) starts the whole sequence all over

I don't know much about Duktape, so I'm not entirely sure how the stack exactly looks like in each step, so please excuse any inaccuracies in my description ;).

One idea to solve this would be to push the variant's string representation to the duktape stack.

Start by tracing the reported call chain from `DuktapeObjectWrapper::getPropertyValue` to `DuktapeScriptBackend::pushToDukStack` while reproducing with `gloperate-qt-example` and `gloperate.canvas0.pipeline.virtualViewport`. Check how custom `Variant` types such as `glm::vec4` are handled and how the Duktape stack changes. Done means evaluating this property no longer recurses or overflows and returns a usable result to the script.

Written by the indexing model from the issue text.