add a "Plasma made by KDE" sign in the splash

Summary: as discussion about branding, as the main logo is changed, add a mention that Plasma is a KDE project in a fine print on the bottom of the screen

Test Plan:
displays correctly in the splash as the attached screenshot
if this is a good change or not i don't have much opinion

Reviewers: Plasma

Subscribers: andreaska, hein, broulik, graesslin, mck182, subdiff, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4050