CaptureMouse in LayoutFloatingWindowControl causes mouse drag to fail when using Expression theme

[juyanith]

PR #367 added a call to CaptureMouse in LayoutFloatingWindowControls.cs. Unfortunately, when using the Expression theme and attempting to drag a floating window that contains a single element by clicking on the tab this causes the application to stop responding to mouse clicks. You then have to click away from the application to restore function. I have created a sample application that demonstrates this problem at https://github.com/juyanith/CaptureMouseError.
This doesn't appear to be an issue using the default theme, perhaps because it does not display the tab when there is only one element contained in the floating window. However, I haven't been able to pinpoint any issues in the theme itself that is causing the problem.