Make logout greeter share QMLEngine

Summary:
Gives a speed boost on multiple monitors where we have multiple views.
Also cleaned up some imports/paths used

Test Plan:
Grepped use of rootContext() referred to the right context
Used greeter to log out successfully

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12891