[kstyle] Tools area

Summary: There is now a tools area.

Test Plan:
{F8132249}

{F8132260} (no border drawn when titlebar and content colours are the same)

{F8132252}

{F8132253}

Reviewers: #plasma, #breeze, #vdg, hpereiradacosta, davidre, ngraham

Reviewed By: #breeze, #vdg, hpereiradacosta, ngraham

Subscribers: mart, maartens, abstractdevelop, IlyaBizyaev, davidre, davidedmundson, hpereiradacosta, ngraham, manueljlin, niccolove, ndavis, plasma-devel

Tags: #plasma

Maniphest Tasks: T10201, T11661

Differential Revision: https://phabricator.kde.org/D27669