Sync scrollbar behavior with GTK applications

Summary:
Previously GTK applications respected their own setting on what should happen when an user clicks below or above scrollbar slider. Now it is in sync with Plasma and Qt applications settings.

Problems:

1. I'm failed to found DConf GTK setting, that is responsible for scrollbar behavior. Maybe someone could help.
2. Setting apply on the go do not work with GTK2 apps, therefore they need to be reloaded to respect the new setting.

Test Plan:

1. Update plasma-desktop and frameworkintegration
2. Launch workspace behavior KCM and change scrollbar setting Clicking on scrollbar track
3. GTK3 apps should now respect the setting

Reviewers: cblack, VDG, ngraham, apol

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10611

Differential Revision: https://phabricator.kde.org/D25703