Default theming for TreeView
Add some new images for combo backgrounds that we will use to simplify the theme. They provide rounded corners on both sides.
Simplify theming of comboboxes. We force combos to draw a shadow that covers the whole widget, prevent children from drawing any shadow and customize backgrounds for team combos. The toggle buttons have a 1x1 background image that should match the combo background color. They can also receive a pixmap with different states if we want but the background colors have to match.
Text color is white in Combobox content.