Add command to switch onion skin visibility

data/gui.xml

@@ -76,6 +76,7 @@
       <key command="Preview" shortcut="F8" />
       <key command="ShowGrid" shortcut="Shift+G" />
       <key command="SnapToGrid" shortcut="Shift+S" />
+      <key command="ShowOnionSkin" shortcut="F3" />
       <key command="Timeline" shortcut="Tab">
         <param name="switch" value="true" />
       </key>
@@ -375,6 +376,8 @@
         <item command="SnapToGrid" text="&amp;Snap to Grid" />
         <item command="GridSettings" text="Gri&amp;d Settings" />
         <separator />
+        <item command="ShowOnionSkin" text="Show &amp;Onion Skin" />
+        <separator />
         <item command="Timeline" text="&amp;Timeline">
           <param name="switch" value="true" />
         </item>

src/app/CMakeLists.txt

@@ -56,6 +56,7 @@ add_library(app-lib
   commands/cmd_new_frame.cpp
   commands/cmd_new_layer.cpp
   commands/cmd_new_layer_set.cpp
+  commands/cmd_onionskin.cpp
   commands/cmd_open_file.cpp
   commands/cmd_open_in_folder.cpp
   commands/cmd_open_with_app.cpp

src/app/commands/cmd_onionskin.cpp

@@ -0,0 +1,67 @@
+/* Aseprite
+ * Copyright (C) 2001-2014  David Capello
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "app/app.h"
+#include "app/commands/command.h"
+#include "app/context.h"
+#include "app/settings/document_settings.h"
+#include "app/settings/settings.h"
+#include "base/compiler_specific.h"
+
+namespace app {
+
+using namespace ui;
+using namespace gfx;
+
+class ShowOnionSkinCommand : public Command {
+public:
+  ShowOnionSkinCommand()
+    : Command("ShowOnionSkin",
+              "Show Onion Skin",
+              CmdUIOnlyFlag)
+  {
+  }
+
+  Command* clone() const OVERRIDE { return new ShowOnionSkinCommand(*this); }
+
+protected:
+  bool onChecked(Context* context)
+  {
+    IDocumentSettings* docSettings = context->getSettings()->getDocumentSettings(context->getActiveDocument());
+
+    return docSettings->getUseOnionskin();
+  }
+
+  void onExecute(Context* context)
+  {
+    IDocumentSettings* docSettings = context->getSettings()->getDocumentSettings(context->getActiveDocument());
+
+    docSettings->setUseOnionskin(docSettings->getUseOnionskin() ? false: true);
+  }
+};
+
+Command* CommandFactory::createShowOnionSkinCommand()
+{
+  return new ShowOnionSkinCommand;
+}
+
+} // namespace app

src/app/commands/commands_list.h

@@ -99,6 +99,7 @@ FOR_EACH_COMMAND(SavePalette)
 FOR_EACH_COMMAND(Scroll)
 FOR_EACH_COMMAND(SetPalette)
 FOR_EACH_COMMAND(ShowGrid)
+FOR_EACH_COMMAND(ShowOnionSkin)
 FOR_EACH_COMMAND(SnapToGrid)
 FOR_EACH_COMMAND(SplitEditorHorizontally)
 FOR_EACH_COMMAND(SplitEditorVertically)