Improve PageRouter API

Summary:
This patch fixes shortcomings in the PageRouter API regarding ease of use, verbosity, idiomaticness, and signals.

Introduced:

• pushFromHere() and popFromHere(): pushing and popping operations that operate from the current route
• watchedRoute and watchedRouteActive: declarative counterpart to routeActive that responds to signals
• Navigation signals

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: mart, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29288