Add SwitchNonactiveLayersOpacity command (fix #1515)

This is a temporal solution but it's good enough for an experimental
feature.

data/strings/en.ini

@@ -370,6 +370,7 @@ SnapToGrid = Snap to Grid
 SpriteProperties = Sprite Properties
 SpriteSize = Sprite Size
 SwitchColors = Switch Colors
+SwitchNonactiveLayersOpacity = Switch Nonactive Layers Opacity
 SymmetryMode = Symmetry Mode
 TiledMode = Tiled Mode
 Timeline = Switch Timeline

src/app/ui/editor/editor.cpp

@@ -16,7 +16,9 @@
 #include "app/color_utils.h"
 #include "app/commands/commands.h"
 #include "app/commands/params.h"
+#include "app/commands/quick_command.h"
 #include "app/console.h"
+#include "app/i18n/strings.h"
 #include "app/ini_file.h"
 #include "app/modules/editors.h"
 #include "app/modules/gfx.h"
@@ -2485,4 +2487,30 @@ void Editor::updateAutoCelGuides(ui::Message* msg)
     invalidate();
 }
 
+// static
+void Editor::registerCommands()
+{
+  Commands::instance()
+    ->add(
+      new QuickCommand(
+        CommandId::SwitchNonactiveLayersOpacity(),
+        []{
+          static int oldValue = -1;
+          auto& option = Preferences::instance().experimental.nonactiveLayersOpacity;
+          if (oldValue == -1) {
+            oldValue = option();
+            if (option() == 255)
+              option(128);
+            else
+              option(255);
+          }
+          else {
+            const int newValue = oldValue;
+            oldValue = option();
+            option(newValue);
+          }
+          app_refresh_screen();
+        }));
+}
+
 } // namespace app

src/app/ui/editor/editor.h

@@ -269,6 +269,8 @@ namespace app {
     // freehand tool is pressed.
     bool startStraightLineWithFreehandTool(const ui::MouseMessage* msg);
 
+    static void registerCommands();
+
   protected:
     bool onProcessMessage(ui::Message* msg) override;
     void onSizeHint(ui::SizeHintEvent& ev) override;

src/app/ui/main_window.cpp

@@ -101,6 +101,8 @@ MainWindow::MainWindow()
   m_previewEditor = new PreviewEditorWindow();
   m_timeline = new Timeline();
 
+  Editor::registerCommands();
+
   m_workspace->setTabsBar(m_tabsBar);
   m_workspace->ActiveViewChanged.connect(&MainWindow::onActiveViewChange, this);