Make TileMap overridable "set_cell" function called on undo/redo

[samdze]

This is a follow-up to #27500.
The set_cell override was working fine in the editor and undo/redo operations were working as always.
The only thing was that the overridden set_cell was not called on undo/redo, the default native set_cell was called in this case.

This PR fixes that issue, making the editor always call the TileMap set_celld method (which is used only in the TileMap editor and is not exposed to scripting languages), even on undo/redo, which then makes the call to set_cell.

set_celld becomes an utility method to make the call to the overridable set_cell method easier.

[MarianoGnu]

renaming that function would be good, not mandatory tho. Changes looks good

[akien-mga]

Thanks!