elm_code_widget: fix select and drag (scrolling).
Currently when selecting with the mouse and scrollling,
the selection will only continue in one direction. With
this patch, the selection can move freely between "pages".
Test Plan: select text and drag up and down in elm_code widget (Edi).
Reviewers: committers, ajwillia.ms
Reviewed By: ajwillia.ms
Subscribers: cedric, zmike
Differential Revision: https://phab.enlightenment.org/D6632