Use the right icon depending on the layout

Summary: If we're showing the icons in a horizontal layout, the < and > buttons feel like it should be selecting instead of switching pages.

Test Plan:
Tested locally

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24408