Create renderer should default to the active dataset

[sgpearse]

It's too easy to overlook the Data Source field in the new renderer dialog. By default it selects the first renderer in the list. If a user has two loaded datasets and wants to create a renderer in the second one, they need to notice and specify this dropdown option.

Instead, if the user has a currently active dataset the Data Source should default to what's selected. In the screenshot below, the wrf3d dataset is active, and should be the target for the new renderer.