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.

Modules/SettingsPanel/Tabs/WallpaperSelectorTab.qml

@@ -135,27 +135,32 @@ Item {
 
             delegate: Rectangle {
               id: wallpaperItem
+
               property string wallpaperPath: modelData
               property bool isSelected: wallpaperPath === WallpaperService.currentWallpaper
 
               width: wallpaperGridView.itemSize
               height: Math.floor(wallpaperGridView.itemSize * 0.67)
-              radius: Style.radiusMedium * scaling
-              color: isSelected ? Color.mPrimary : Color.mSurface
-              border.color: isSelected ? Color.mSecondary : Color.mOutline
-              border.width: Math.max(1, Style.borderThin * scaling)
-              clip: true
+              color: Color.transparent
 
-              NImageRounded {
-                anchors.fill: parent
-                anchors.margins: Style.marginTiny * scaling
+              // 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.
+              NImageCached {
+                id: img
                 imagePath: wallpaperPath
-                fallbackIcon: "image"
-
-                imageRadius: Style.radiusMedium * scaling
+                anchors.fill: parent
               }
 
-              // Selection indicator
+              // Borders on top
+              Rectangle {
+                anchors.fill: parent
+                color: Color.transparent
+                border.color: isSelected ? Color.mPrimary : Color.mOutline
+                border.width: Math.max(1, Style.borderThick * scaling)
+              }
+
+              // Selection tick-mark
               Rectangle {
                 anchors.top: parent.top
                 anchors.right: parent.right