add history to layer create, delete, move, rename, and merge

src/canvas_layer.cpp

@@ -1,6 +1,8 @@
 #include "pch.h"
 #include "canvas_layer.h"
 
+uint32_t Layer::s_count = 0;
+
 void Layer::Snapshot::create(int w, int h)
 {
     for (int i = 0; i < 6; i++)