Fix PgUp/PgDn on layer stack

Switch the activation flags for activateNextLayer and activatePreviousLayer from KisAction::ACTIVE_LAYER to KisAction::ACTIVE_NODE.
ACTIVE_LAYER makes these actions only activate on nodes that inherit from KisLayer, which results in incorrect behavior when the active node is a mask.

BUG: 369443
Differential Revision: https://phabricator.kde.org/D3010