New yakuake logo/icon

Summary:
Split from: https://phabricator.kde.org/D24621

This is my proposition for new yakuake logo and icon. The logo mixes a
well known terminal symbol (>_) and letter "y".

Reviewers: VDG, Yakuake, hein, ndavis, mweepigeon

Reviewed By: VDG, ndavis, mweepigeon

Subscribers: mweepigeon, davidre, GB_2, ndavis, ognarb, ngraham, hein, kde-frameworks-devel

Tags: Frameworks, Yakuake

Differential Revision: https://phabricator.kde.org/D25123