[KColorschemeManager] Add option to reenable following global theme

Summary: We need remove the titlebar hint property and simply go back to standard palette.

Test Plan:
Select "System color scheme" in kcolorschemedemo/kate, change scheme in colors kcm

(Ignore the duplicated themes)

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: ahmadsamir, asemke, kossebau, ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12147

Differential Revision: https://phabricator.kde.org/D25877