Add context menu options for editable palette in Canvas; implement BeginCanvas and EndCanvas functions for improved canvas management.

src/app/gui/canvas.h

@@ -171,7 +171,7 @@ class Canvas : public SharedRom {
   auto width() const { return canvas_sz_.x; }
   auto height() const { return canvas_sz_.y; }
   auto set_draggable(bool value) { draggable_ = value; }
-
+  auto canvas_id() const { return canvas_id_; }
   auto labels(int i) {
     if (i >= labels_.size()) {
       labels_.push_back(ImVector<std::string>());
@@ -245,6 +245,9 @@ class Canvas : public SharedRom {
   std::vector<ImVec2> selected_tiles_;
 };
 
+void BeginCanvas(Canvas &canvas, ImVec2 child_size = ImVec2(0, 0));
+void EndCanvas(Canvas &canvas);
+
 void GraphicsBinCanvasPipeline(int width, int height, int tile_size,
                                int num_sheets_to_load, int canvas_id,
                                bool is_loaded, BitmapTable &graphics_bin);