Fix updates on autokeyframing with onion skins enabled
When we create a new frame, we may change a set of onion skins that should be displayed on screen. Therefore, we should issue a set of specific updates for such a case.
The patch adds a set of more granular signals to KisKeyframeChannel, so that the addition and removal of the keyframes can be tracked properly.
The patch also fixes the issue with sigRemovingKeyframe() being emitted after the frame was actually removed from the raster channel.