Move "Full Screen Mode" item from Settings menu to View menu

Summary:
This is a pretty longstanding feature request and all the HIG and VDG people polled
were in favor. Since those were the people requested to sign off on it, it looks like
we can make the necessary code change now.

BUG: 106807
FIXED-IN: 5.63

Test Plan:

Reviewers: VDG, Frameworks, cfeck, ndavis

Reviewed By: VDG, ndavis

Subscribers: dfaure, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24091