Blender Git Loki

UI: Click modifier icon to set active modifier

This should be a final piece of the changes for the active modifier
interface. Before, it was necessary to click on the blank space of a
modifier panel to set it active (not the header), this commit allows
clicking on the icon also.

The spacing with the spacing with the expand button would ideally
be a bit larger, but the layout system doesn't offer much flexibility
here.