NavigationTabBar: Use the standard ToolBar background from the style

Components should never have a custom style, but should follow whatever the qqc style uses, especially thins which is for all intents and purposes in the bottom toolbar area

Probably on the mobile qqc2-breeze-style toolbars should have a small shadow instead.

Fixes #70 (closed)

This is a multi part, as the API for this is still a bit problematic and before freeze i would like to remove the whole of the color API as well (and use only the theme attached property for that)