Make startplasma more flexible so it can be leveraged by plasma mobile

Summary:
Allows specifying the shell through an environment variable.
Allows forwarding command line arguments to kwin_wayland, since the arguments we
use on plasma phone are slightly different.

Test Plan: Builds, still starts

Reviewers: Plasma, bshah, mart, davidedmundson

Reviewed By: Plasma, bshah, mart, davidedmundson

Subscribers: broulik, mart, alexeymin, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25750