Summary:
currently, pos/vol could be changed by using only knob not slider area.
use slider area as event source, and control knob internally.
Fixes T1440
Test Plan: terminology -> tycat video file -> try to control pos/vol by clicking slider area
Reviewers: billiob
Subscribers: seoz
Maniphest Tasks: T1440
Differential Revision: https://phab.enlightenment.org/D1219