Trying to merge layers into the timeline, and eventually add more features like "share layer with all frames", among others. THIS IS NOT FINISHED, IT WILL *NOT* WORK PROPERLY. Once it is finished, this branch will be merged onto master.
So far only add layer and add frame work, and even they may have some issues. Undoing also does not work properly yet.
The UI is also not finished, as it currently has problems with the scroll containers.
