fix showing of window on inactive activities during session save

This is what 7ce38049 tried to do (and got reverted by my previous commit).
The approach is basically the same, just temporarily fake that the window
is on all activities, thus it will be temporarily shown for the session
interact, but this time there seem to be no broken side-effects (and if there
are, they'll be at least limited to activities and not break virtual desktops).
Also, 'needsSessionInteract' was a misnomer - it certainly wasn't set for all
windows that needed session interaction. Just call it what it really is.

Differential Revision: https://phabricator.kde.org/D5614