Port Plasma Desktop Scripting to QJSEngine

Summary:
Port plasma desktop scripting to qjsengine, removing every trace of QScript from plasma-workspace.
The scripting is 100% API compatible.
the bindings are partly done on the javascript part as some things are only possible there and not on the QJSEngine part:

• setting AppInterface as the globalobject's proto with property getters that are evaluated every time
• setting functions that act as constructors (using newQMetaObject gives way more complex and uglier c++ code in Containment, as the scriptengine is not immediately accessible)
• a QRectF wrapper which exposes all properties and methods that were exposed in the previous implementation

Test Plan:

• some manual api test from the interactive console
• tested both from the desktop console and first start with some pretty complex layouts from lnf packages (the default one, netrunner, united, elpas)
• tested every property/methos of the qrectf wrapper

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13112