Resurrect show tooltips option

Summary:
In KDE 4, there was a very handy option to disable decoration tooltips.
Decoration tooltips were lost in transition to KDE Plasma 5, and so
the option.

Given that decoration tooltips were brought back to KDE Plasma 5, "Show
decoration button tooltips" option can be still useful for people(like me)
who may wish to disable them because of personal preference.

Reviewers: #kwin, broulik, mart

Reviewed By: #kwin, mart

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19152