Fix gtk-primary-button-warps-slider with GTK 2

Summary:
Replace true/false with 1/0. The former works only with GTK 3, but
breaks with GTK 2 apps.

BUG: 376273
FIXED-IN: 5.8.6

Test Plan: Tested with inkscape (which is gtk2); still works with gtk3 apps.

Reviewers: apol, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4557