You can add tags by right clicking on a cel and selecting "Add Frame Tag". No tag modifying and deletion is implemented yet.
I'm using a different ScrollContainer for the tags, which gets updated everytime the main timeline ScrollContainer gets updated. I hide its sliders from its theme, wish there was a more straightforward way.
The onion skinning button replaces the old onion skinning area of the timeline, but there's no way right now to change the settings. This will be added in a future commit. More UI changes will soon follow.
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.