Fix incorrectly using None interpolation type
    
    Note: Original fix ported from Tahoma
    
    This PR fixes an problem in Tahoma that also exists in the Official OpenToonz version noted in issue opentoonz/opentoonz#3168
    
    When a new keyframe is created, the keyframe may inadvertently be created as a None interpolation type. There may be multiple ways to do this, but 1 known way is to create a new keyframe before the 1st keyframe in the column and then copy/paste the newly created keyframe after it. This was due to not updating m_prevType in the next keyframe properly.
    
    A None interpolation keyframe is written to the scene file as <n>...<\n> which can lead to a crash while loading the scene or create strange behavior with the keyframes in the column.</n>
    
    This fix should
    
    Prevent the creation of a None interpolation type keyframe
    
    Throw out None interpolation keyframes when loading the scene in case it encounters one.
    
    Co-Authored-By: manongjohn <19245851+manongjohn@users.noreply.github.com>