TasksModel: cache launcherCount().

Summary:
This also allows to only emit launcherCountChanged() when it actually changed.

The emit from TasksModel::filterAcceptsRow() is weird though.

Test Plan:
adding one launcher for dolphin, shows up. Running dolphin,
the launcher disappears. Switching desktops, it reappears. Coming back, it
disappears. Closing dolphin, the launcher reappears.

Reviewers: hein

Reviewed By: hein

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1865