Scripting: Fixed UI issues after setting class values

[bjorn]

When setting a class value from a script, the value was not getting converted from a JS object to a QVariantMap. This tripped up code elsewhere, for example making the UI unable to edit the values (though it could still display them, and they also got saved correctly).

This change adds the missing conversion from JS object to QVariantMap to tiled.propertyValue and Object.setProperty.

[bjorn]

Even if this PR adds a convenience function Object.setProperty, I've passed on adding an equivalent Object.property overload that takes a path for now. This functionality doesn't exist on the C++ side either at the moment, so it would be a larger patch, and the usefulness is less obvious since it just makes this sequence a bit less verbose:

// currently supported
let nestedPropertyValue = object.property("foo").value.bar.value.baz;

// vs:
let nestedPropertyValue = object.property(["foo", "bar", "baz"]);

In addition, we'd then also need to add path support to Object.resolvedProperty.

There was no need for overloading Object.setColorProperty or Object.setFloatProperty, because class members have their type defined by their class. The setProperty overload taking a path will try to convert the values to the type of the class member (though this currently isn't the case of top-level properties).

Object.setColorProperty has been deprecated in favor of using the new tiled.color to create a color value. Unfortunately I couldn't immediately find a quick way to do the same with Object.setFloatProperty.