[plasma-framework] port away from deprecated KF5 APIs

Summary:
KDeclarative::setupBindings() is deprecated, replaced calls with its
implementation (setupContext + static setupEngine call), but see comment
about whether we can be sure this is done only once per engine, as
requested by the setupEngine API. It's 100% equivalent to the old code
though, can't be worse.

KPackage::contentsHash is deprecated, replaced call with implementation.

Depends on D25433 for the KActionCollection::changed signal.

Test Plan: Builds

Reviewers: mart, davidedmundson, broulik, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25434