Window Style Part 1 of 3

.. refactor linear gradients out of the widgets
   such as toolbar, chartbar, sidebaritem etc

.. GCColor::linearGradient() now returns a gradient
   that is reused across the codebase

.. in Part 2 we will add a colour and flat style
   for the linearGradient

.. in Part 3 we will tidy up the styling for buttons
   and toolbuttons to adopt flat or metal styling too