Issue Summary

This is something more of a user experience issue that has been requested here and there. Onion skinning in most programs is normally turned on / off reminiscent of how a light table would. After that you are to handle the inclusion / exclusion of either previous or next frames relative to your current frame.
Consider this I'd like to ask to have a single switch (and shortcut) for toggling the onion skin globally, and leave the current PREV and NEXT functionality to be perceived as filters. This way there would be the following Onion Skin states: All OFF, All ON, PREV ONly, NEXT ONly

So basically we would end up having:

• A global switch to turn onion skin ON / OFF (w/ shortcut)
• The PREV onion skin switch (w/shortcut)
• The NEXT onion skin switch (w/shortcut)