Revert "[GTK3] Revert checkbox recolouring"

Summary:
This reverts commit b1649126, as an upstream librsvg bug was fixed, allowing checkboxes and radiobuttons to recolour properly.

https://gitlab.gnome.org/GNOME/librsvg/issues/525

Test Plan: See that checkbox recolouring works as expected.

Reviewers: #breeze, #vdg, ngraham

Reviewed By: #breeze, #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25336