* New keybind: 'v' to open a new Viewport in the Level Editor.
* New keybind: Backspace to close the topmost UI window,
and Shift+Backspace to close them all.
* Zoom has graduated out of experimental feature status. Still a bit
buggy but workable.
* Viewport windows now copy the Tool and BrushSize of the toplevel
editor, so drawing in and out of viewports works well.
* Viewport window UI improved: buttons to grow or shrink the window
size, refresh the actors, etc.