Add a PixmapButtonDelegate column to VariationTree.
Change ItemDetailsWidget's window flag from Dialog to Tool to make it
always flow on top of the main window.
Place TypeTree's loading into a separate thread.

ItemIndexDock will now use default categorization method.
Rearrange main.cpp, now mainWindow will be shown after all docks are
ready.
Add BusyBar.
A BusyBar is displayed when MarketOrdersWidget is fetching orders.
Replace setButtonState with setBusy in MarketOrdersWidget.