Move wallpaper() function to a correct file.

src/colorscheme/ColorScheme.cpp

@@ -537,3 +537,8 @@ void ColorScheme::setWallpaper(const QString &path)
 {
     _wallpaper = new ColorSchemeWallpaper(path);
 }
+
+ColorSchemeWallpaper::Ptr ColorScheme::wallpaper() const
+{
+    return _wallpaper;
+}

src/colorscheme/ColorSchemeWallpaper.cpp

@@ -17,11 +17,6 @@
 
 using namespace Konsole;
 
-ColorSchemeWallpaper::Ptr ColorScheme::wallpaper() const
-{
-    return _wallpaper;
-}
-
 ColorSchemeWallpaper::ColorSchemeWallpaper(const QString &path) :
     _path(path),
     _picture(nullptr)