D8564: Disallow drop of task manager icons outside of plasmoid when widgets are locked

[Nathaniel Graham]

ngraham added a reviewer: VDG.

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D8564

To: emateli, plasma-devel, #plasma, #vdg
Cc: ngraham, davidedmundson, romangg, hein, plasma-devel, ZrenBot, progwolff, 
lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart