[Kicker] Only show "Add to Panel (Widget)" When there's no Task Manager

Summary:
Users get confused by the difference between "Pin to Task Manager" and "Add to Panel (Widget)", and we get bugs about this.

This patch resolves that confusion by only showing "Add to Panel (Widget)" when there's no Task Manager widget.

BUG: 390817
FIXED-IN: 5.14.0

Test Plan:
Dragging-and-dropping an app from Kicker or Kickoff pins it to the Task Manager if there's a Task Manager, or creates a Panel launcher if there's no Task Manager.

Context menu items:

Task Manager, widgets unlocked:

Task Manager, Widgets locked:

No Task Manager, Widgets unlocked:

Task Manager, Widgets unlocked:

Reviewers: Plasma, davidedmundson, mart

Reviewed By: Plasma, mart

Subscribers: mart, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12969