Change Background Image for games (#2334)

* Added opacity change instead of blur for background image

* Fixed integer overflow when refreshing grid list

* Added slider to control background image opacity

* Added show background image button

* Added UI code for checkbox and English and Spanish translations for new UI elements

* Removed background image caching

* Background image update on apply/save

* Only recompute image if opacity or game changes

* Fixed segfault when trying to change opacity after table refresh

* Placed background image settings under GUI in settings file

src/qt_gui/settings_dialog.h

@@ -33,6 +33,7 @@ public:
 signals:
     void LanguageChanged(const std::string& locale);
     void CompatibilityChanged();
+    void BackgroundOpacityChanged(int opacity);
 
 private:
     void LoadValuesFromConfig();