Another Scrollbar ..

.. AllPlot IntervalSummary overlay needs its
   scrollbar styled.
2
Translation: Update for 3.1
Fixup Metadata Styling on Linux

.. by using the Fusion style on the widgets
Translation Enablement + Check on temporary Seasons

... Translation for Season Types
... Do not allow Temporary Seasons to be edited or deleted through the
main seasons menu (in right click Popup this was already blocked by not
offering the function)

(cherry picked from commit cc7a0a7f8d0b74ca4b9d64d61291206ea58d3f94)
Merge pull request #896 from Joern-R/pull

Translation Enablement + Check on temporary Seasons
RideEditor Cosmetics

.. scrollbars tidied up
.. ugly grid lines removed altogether
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
Window Style Part 2 of 3

.. apply Flat styling across all the widget painting

.. only use WHITE as the flat color for now, in part 3
   we will look at user customisation and updating the
   toolbar and sidebar buttons to also be flat.
Windows Style Part 3 of 3

.. flat colors based upon windows prefrences if on
   windows -- BUT ONLY if Dsmapi.h is available.
Merge Upstream Master