Make sure the background of the PreviewCanvas is drawn

This PR should fix the problem with the distorted PreviewCanvas @gusferguson noticed today.
Test: Open the Windows(!) build of Insight, select an Image, change some settings in the rendering settings; make sure that the preview image is shown neatly (i. e. no weird drawing artifacts in the background).
I don't know for sure if this fixes the problem, as I can't create a Windows build at the moment, but I strongly assume that the problem was the combination of double buffering and the overwritten paintComponent method (although I'm surprised, that this does not show up on OSX).
/cc @pwalczysko @jburel