Fix another regression with rearranging launchers in an activities world.

Summary:
The new activity-aware implementation of
LauncherTasksModel::setLauncherList() would only accept the passed list
when it changed any activities associations, not when the order changed.
This would effectively turn TasksModel::move involving launcher tasks
into a no-op.

Rearranging launchers works like this:
1. A client calls TasksModel::move one or many times. TasksModel::move
   updates TasksModel's internal sort mapping and implements it, causing
   a visible order change in any views.
2. When it is done calling move(), the client calls
   TasksModel::syncLaunchers. TasksModel::syncLaunchers calls
   LauncherTasksModel::setLauncherList with a new list derived from
   its sort mapping, and updates its sort mapping in expectation of
   row indices changing in the launcher tasks source model.

Due to the above bug, the sort mapping would be adjusted in expectation
of changes the launcher tasks model would not actually do, appearing to
undo the moves the next time a view is updated from source data.

CCMAIL:ivan.cukic@kde.org

Reviewers: #plasma, davidedmundson, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4749