Timeline: fix crash when we remove all layers (issue #393)
Timeline: fix painting code for paddings area when we've removed all layers
Fix NewFrame command so it's enabled even when we don't have layers
Fix RemoveLayer command so it's disabled when we have no more layers to remove
AppMenuItem: fix updating enabled/checked state in popup menus (in timeline popups mainly)
Fix Clear command to delete the selection first when the range is enabled in one frame/one layer
Update TODO.md
Merge branch 'dev' of git@github.com:aseprite/aseprite.git into dev