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/Sidebar/Panel/Music.qml

@@ -93,6 +93,7 @@ Rectangle {
                             anchors.margins: 2
                             fillMode: Image.PreserveAspectCrop
                             smooth: true
+                            mipmap: true
                             cache: false
                             asynchronous: true
                             sourceSize.width: 60

Widgets/Sidebar/Panel/System.qml

@@ -60,8 +60,8 @@ Rectangle {
                             fillMode: Image.PreserveAspectCrop
                             asynchronous: true
                             cache: false
-                            sourceSize.width: 44
-                            sourceSize.height: 44
+                            smooth: true
+                            mipmap: true
                         }
                         maskSource: Rectangle {
                             width: 44

Widgets/Sidebar/Panel/WallpaperPanel.qml

@@ -137,8 +137,8 @@ PanelWindow {
                                     fillMode: Image.PreserveAspectCrop
                                     asynchronous: true
                                     cache: true
-                                    sourceSize.width: Math.min(width, 150)
-                                    sourceSize.height: Math.min(height, 90)
+                                    smooth: true
+                                    mipmap: true
                                 }
                                 MouseArea {
                                     anchors.fill: parent