Use a native application for starting plasma

Summary:
At the moment we had several scripts to start the different processes. With this we unify this code into an application that takes care of the whole process.
This allows us to:
- Save on process spawning, we don't need to run a separate process synchronously for every single thing.
- Don't have a redundant configuration file parser but reuse the one we've already optimised in KConfig.
- Issue dbus calls from the process itself instead of spawning qdbus.
- Removes a bunch of duplicated code.

Test Plan: Started different systems on different distros, on wayland and x11.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21725