Main: more robust error handling.

Previously, the editor would panic at any error. After I recently
implemented dialogs, I changed all panics to create error dialogs,
instead. This will make the editor more stable.

ui/component.go

@@ -10,7 +10,7 @@ import (
 //
 // Many components implement their own `New...()` function. In those constructor
 // functions, it is good practice for that component to set its size to be its
-// minimum size. TODO: implement that behavior
+// minimum size.
 type Component interface {
 	// A component knows its position and size, which is used to draw itself in
 	// its bounding rectangle.