Fix panelView updateEnabledBorders being skipped

Summary:
Sometimes the left/right panel edge shadows don't get drawn.
A resize doesn't always call positionPanel so it never gets updated on all
size changes.

Putting it in the event handlers rather than in the setting, means it
will never be missed again.

Test Plan:
Set panel to fill width
Resizing now shows correct corner

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2668