Respect icon.height and width in custom button contents

Signed-off-by: Claudio Cambra <claudio.cambra@nextcloud.com>

src/gui/tray/NCButtonContents.qml

@@ -24,8 +24,8 @@ RowLayout {
     property bool hovered: false
     property string imageSourceHover: ""
     property string imageSource: ""
-    property int imageSourceWidth: 64
-    property int imageSourceHeight: 64
+    property int imageSourceWidth: undefined
+    property int imageSourceHeight: undefined
     property string text: ""
     property var display