I feel this can be a hassle to code, so if you can't do it now or even at all, then no worries; at least the undo/redo buttons will be enough.
I would like to know if you can make it so you can set a terrain/object to prevent it from being moved. You can right-click on a terrain/object piece to bring up a menu, then click "Lock piece". This is to prevent you from moving the piece at all if you attempt to drag it, until you ask to have it "unlocked".
This, again, will be very useful in if you want to modify someone's level but don't want to move any of the terrain/objects. It will also be useful for tag-team contests where one cannot move the terrain/objects.
This is probably the easier of the two to implement. I'll look into it when I get a chance (probably tomorrow).
In regards to this - should the locked / unlocked status also be saved into the level file? (No change would be needed to the player to accomodate this inherently, although unless changes to the player are made, the flag wouldn't be applied in dumped levels.) Or should it be purely in the editor's memory, and thus lost when the editor (or even just the level) is closed?
Aside from potentially saving this to a file, this has now been implemented. :)
For now, I implemented support for including lock / unlocked status in the saved level file (yes, this is supported for SuperLemmini levels too; no, it doesn't at all interfere with their playability in SuperLemmini itself). If this turns out to be annoying, I can change it later.