Use symbolic icon in intro page

Summary:
Sorry for the large amount of smaller changes but on dark themes the right ">" is too dark to be noticeable - just look at the screenshots:

old:
{F7388372}

new:
{F7388375}

on light theme:
{F7388377}

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24097