Adjust titlebar style for Tools Area and new color scheme

Merged Nate Graham requested to merge ngraham/breeze:toolsarea-titlebar-changes into master

The titlebar gradient is removed and the close button is given a red background.

This supports https://phabricator.kde.org/T10201 and is intended to be used with !2 (merged) (new color scheme) and https://phabricator.kde.org/D27669 (Tools area appearance)

vokoscreenNG-2020-04-01_11-38-04

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