gui: use an Image item to set the on/off icon

this uses the contentItem property of RoundButton to
use an Image item and set the power button as on/off
changing from Icon to Image allows to use the mipmap
property, which should improve the image quality

gui/components/StatusBox.qml

@@ -151,11 +151,10 @@ Item {
             HoverHandler {
                 cursorShape: Qt.PointingHandCursor
             }
-            icon {
+            contentItem: Image {
+                anchors.fill: parent
                 source: Theme.buttonDisconnected
-                height: 48
-                width: 48
-                color: "transparent"
+                mipmap: true
             }
 
             onClicked: {

gui/components/VPNState.qml

@@ -63,7 +63,7 @@ StateGroup {
             PropertyChanges {
                 target: toggleVPN
                 enabled: false
-                icon.source: Theme.buttonConnecting
+                contentItem.source: Theme.buttonConnecting
             }
             PropertyChanges {
                 target: systray
@@ -94,7 +94,7 @@ StateGroup {
             PropertyChanges {
                 target: toggleVPN
                 enabled: true
-                icon.source: Theme.buttonDisconnected
+                contentItem.source: Theme.buttonDisconnected
                 Accessible.name: qsTr("Turn on")
             }
             PropertyChanges {
@@ -130,7 +130,7 @@ StateGroup {
             PropertyChanges {
                 target: toggleVPN
                 enabled: true
-                icon.source: Theme.buttonConnected
+                contentItem.source: Theme.buttonConnected
                 Accessible.name: qsTr("Turn off")
             }
             PropertyChanges {
@@ -168,7 +168,7 @@ StateGroup {
             PropertyChanges {
                 target: toggleVPN
                 enabled: true
-                icon.source: Theme.buttonConnecting
+                contentItem.source: Theme.buttonConnecting
                 Accessible.name: qsTr("Cancel")
             }
             PropertyChanges {
@@ -207,7 +207,7 @@ StateGroup {
             }
             PropertyChanges {
                 target: toggleVPN
-                icon.source: Theme.buttonConnecting
+                contentItem.source: Theme.buttonConnecting
                 Accessible.name: qsTr("Cancel")
             }
             PropertyChanges {