[Kickoff / Kicker] Request closing plasmoid properly

In 0d6792c I turned the JS into a singleton but overlooked that it also accesses plasmoid context property.
Instead of randomly calling outside objects from this library function,
signal what the caller should do.
The function already returned undefined for doing nothing,and true for requesting a close.
Changed all call sites of this function to explicitly check for true

Differential Revision: https://phabricator.kde.org/D10915