Wallpapers: added thumbnail caching. Had to drop the rounded borders for it to work.

NImageCached relies on the image being visible to work properly.
MultiEffect relies on the image being invisible to apply effects.
That's why we don't have rounded corners here, as we don't want to bring
back qt5compat.

Commons/Settings.qml

@@ -17,6 +17,7 @@ Singleton {
                                                                          "HOME") + "/.config") + "/" + shellName + "/"
   property string cacheDir: Quickshell.env("NOCTALIA_CACHE_DIR") || (Quickshell.env("XDG_CACHE_HOME") || Quickshell.env(
                                                                        "HOME") + "/.cache") + "/" + shellName + "/"
+  property string cacheDirImages: cacheDir + "images/"
 
   property string settingsFile: Quickshell.env("NOCTALIA_SETTINGS_FILE") || (configDir + "settings.json")
 
@@ -37,6 +38,7 @@ Singleton {
       // ensure settings dir exists
       Quickshell.execDetached(["mkdir", "-p", configDir])
       Quickshell.execDetached(["mkdir", "-p", cacheDir])
+      Quickshell.execDetached(["mkdir", "-p", cacheDirImages])
     }
   }