Add non-linear undo/redo design documentation and improve UndoSystem with backspace batching and GUI integration fixes.

main.cc

@@ -97,12 +97,11 @@ main(int argc, const char *argv[])
 	} else if (req_term) {
 		use_gui = false;
 	} else {
-
-	// Default depends on build target: kge defaults to GUI, kte to terminal
+		// Default depends on build target: kge defaults to GUI, kte to terminal
 #if defined(KTE_DEFAULT_GUI)
-	use_gui = true;
+		use_gui = true;
 #else
-	use_gui = false;
+		use_gui = false;
 #endif
 	}
 #endif