Improve images resizing quality accross the board

- Using mipmap: true and smooth: true for images that are downscaled
(avatar, icons, ...)
- Using mipmap: false and smooth: true for images that are potentially
upscaled (background, overview, ...)

Widgets/Dock.qml

@@ -155,6 +155,7 @@ PanelWindow {
                             anchors.centerIn: parent
                             source: taskbar.getAppIcon(modelData)
                             smooth: true
+                            mipmap: true
                             visible: source.toString() !== ""
                         }