Emit signals when a package is installed/uninstalled

Summary:
This is needed for situations such as Plasma reloading the wallpaper
plugins when one is installed.

We want something with more granularity than arbitrarily reloading when
the KNS dialog closes, and less horrible than file watchers.

The service type (for example Plasma/Wallpaper) makes up part of the
path for optimal filtering.

Test Plan: Ran dbus-monitor and kpacktool5 -r / -i

Reviewers: Plasma

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15177