Fix bug where QuickDialogTableView does not use root element's appearance for backgroundColor

If the appearance had a nil value for tableGroupedBackgroundColor, the backgroundColor was not being set in applyAppearanceForRoot:. This may be the case when only plain tableViews are used.
Add default backgroundColor values for each UITableStyle to QAppearance

- tableBackgroundColor default value is now white
- tableGroupedBackgroundColor default value is now white
Correctly set backgroundColor in Sample application's loginController tableView

- Overrided initWithRoot: in LoginController to update the rootElements appearance object
- Changed the background colour to blend with the header image