Merge signal connections for AbstractClient in Workspace

Summary:
Have one dedicated method which performs the connection for both
Client and ShellClient. This fixes the desktopPresenceChanged signal
not being passed to the effects.

Note that not all signals are merged. Most signals setup for Client
don't make sense for ShellClient as ShellClient cannot block composite
or unredirect.

Test Plan:
Test case added for ShellClient to ensure that the signal
is correctly invoked on the ShellClient, Workspace and EffectsHandler.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2059