Screen-Scaling breaks tree-view

[RHuehne]

Describe the bug
When Windows is not used in native resolution, but scaled Xen Center tree view pane does not stick to it's outer frame

To Reproduce

1. Set Windows Custom Scaling to 125% (seen on different Machines, especially when using RDP)
2. Start XenCenter

Expected behavior
The tree-view in the left pane uses the whole pane.