Improve the look of the Emoticons KCM UI

Summary:
Apply the KDE HIG, make buttons consistent and make the KCM look better.
{F6619998}

Test Plan: Open the Emoticons KCM.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19093