Replace Kopete icon with the official one.

PNG files in medium, medium2x and original sizes created from the SVG retrieved from the repository:
https://projects.kde.org/projects/kde/kdenetwork/kopete/repository/revisions/081e189e0b6be4c9cd74a7c91120f8ebeee32501/entry/kopete/hisc-apps-kopete.svgz
Merge pull request #785 from selevt/master

Replace Kopete icon with the official one