Don't break compatibility with old configs

Summary:
Commit 4a06c4d3 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