Whoops, thanks for asking about this because actually you're not
supposed to be able to edit animations in .tsx files (well, of course it
would be nice, but this is currently unsupported). Just like with
adding tile properties, it needs to be done while the tileset is part of
the map file and afterwards the tileset should be exported to the .tsx
file.
The root of this issue is that there
is no real .tsx file handling implemented yet and they do not have an
undo stack. The saving of the .tsx file happens only on doing an export,
so any changes made while the tileset is not imported are lost. For
this reason I've tried to make sure any editing functionality in
disabled in the UI for external tilesets, but I forgot to do this for
the animation editor. I'll fix this before doing the 0.10 release.
Best regards,
Bjørn