VST plugin MDI subwindow uses default qt styling

Version `1.1.90-gdaedb92 (Linux/x86_64, Qt 4.8.6, GCC 4.8.4)`, Elementary os Freya x64
![screenshot from 2016-06-05 22 42 13](https://cloud.githubusercontent.com/assets/6282045/15808103/f6f73d2a-2b6e-11e6-892d-fd1c5774ce06.png)
VST instrument windows seem to ignore the custom MDI Subwindow styling. 

Tagging @BaraMGB 
