Various fixes for scrolling on Windows.
Use separate scroll bar and pane windows.
Fix panel item widths.
Improve handle thumb track and scroll change events.
Various fixes for tweet mouseovers on Windows.
Reparent mouseovers to/from their temporary owner, use clip children window style.
Fix mouseovers being hidden when clicking it produced a popup menu.
Add a hook to inc/decrement the mouseover mouse-enter ref count during a popup.