New Welcome Page

The current Welcome Page (kwelcomepage.cpp, kwelcomepage.h) is very plain and could be designed to be more compelling, especially that it is the first thing presented to a new user.

This can be made to match the contents of what would be a revamped, visually-compelling and more concise "Features" page of KMM website.

It should be rewritten using a modern QtQuick/QML, for a future adaptation of the whole UI to Kirgami/Maui.