TODO feature: Undo/Redo for canvas changes

TODO.txt

@@ -1,5 +1,6 @@
 Application:
 V Migrate from .Net Framework 4.7 to .NET 9
+V New Save file format! Use McBitFont v1.7 to convert old save files to the new format.
 V Better quality pictures in symbol list
 
 Functionality:
@@ -8,7 +9,7 @@ V Delete symbols before/after selected
 - Shift all symbols on code line (change symbol codes)
 - Specify starting code (extends the shift)
 V Ability to make monospaced font a variable width one
-- Undo/Redo
+V Undo/Redo for canvas changes
 - Image import from a file
 - Import from a text array
 - Rectangle selection to mass-paint, shift and mirror pixels