Add plasmashell --replace

Summary:
Calls the existing quit signal on a running plasma then tries to start
in the normal way.

It's meant partly to make my own development workflow a bit easier, but
also to make it one less line on bugzilla when I want people to give me
debug output.

Test Plan:
Ran with existing plasma running
Ran without an existing plasma running

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12092