Adjust Latte Dock to libtaskmanager API changes for Wayland

Summary:
Changes in libtaskmanager in Plasma 5.15 currently
break several Latte Dock features.
This patch ensures that Latte Dock will continue to work when Plasma 5.15 is released.

BUG: 403358

FIXED-IN: 0.8.5

Reviewers: mvourlakos

Reviewed By: mvourlakos

Subscribers: plasma-devel

Tags: #latte_dock, #plasma

Differential Revision: https://phabricator.kde.org/D18375