Plasma controls based on QtQuickControls2

Summary:
This is a basic styling of a QtQuickControls2 series based on Plasma theme.
it has the main controls available in Qt 5.7
it installs them as a separate import (org.kde.plasma.controls 3.0) for
use restricted to plasmoids, is probably needed to be still installed as
a style as well

Test Plan:
tried with minimal QML files, a more comprehensive gallery may be needed.

pending considerations:

• some of the classes, like Drawer, the dialogs and ApplicationWindow *don't* make sense in plasmoids
• it will probably still need to be installed also as a qqc2 style, as this should be used in Plasma mobile
• probably only some of the controls should be installed as a separate import, the whole set as a style strictly for Plasma Mobile use. the same qml files would be used, so no maintainance overhead

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4508