Show attribute names in SG geometry tab.
Add a way to trigger sg visualize overdraw feature

The Qt Quick scene graph has a feature to draw a 3D box visualizing
the drawn items in layers, helping to find overdraw. This commit adds
a button to the gammaray quick-inspector to trigger this feature.

Moreover with this commit the quickinspector is able to tell the
client, what kind of features it supports and the client now hides the
appropriate UI if a feature is not supported (e.g. the Scene Graph tab
when the scene graph inspecting feature is not available).
Print warning if signal argument has unknown type.

When trying to expose a signal over the network, gammaray's
multisignalmapper now prints a warning when the type of an argument is
null (== unknown). Prior it was only printed when the argument type
was void.