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

[Emirald Mateli]

emateli added a subscriber: plasma-devel.

REPOSITORY
  R119 Plasma Desktop

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

To: emateli, plasma-devel
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart