Don't break compatibility with old configs

Summary:
Commit 4a06c4d34f doesn't take into account old config files where the
value of the key is 'true', breaking the getter for the warps property.

Test Plan:
Add gtk-primary-button-warps-slider=true both in ~/.gtkrc-2.0 and
~/.config/gtk-3.0/settings.ini; open gtk-config kcm, warps checkbox is now checked
as expected.

Reviewers: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4594