Fix QGCTabBar icon support

src/QmlControls/QGCTabButton.qml

@@ -1,5 +1,7 @@
-import QtQuick                      2.11
-import QtQuick.Controls             2.4
+import QtQuick                      2.12
+import QtQuick.Controls             2.12
+import QtQuick.Controls.impl        2.12
+import QtQml                        2.12
 
 import QGroundControl               1.0
 import QGroundControl.Palette       1.0
@@ -7,17 +9,25 @@ import QGroundControl.Controls      1.0
 import QGroundControl.ScreenTools   1.0
 
 TabButton {
-    id: control
+    id:             control
+    font.pointSize: ScreenTools.defaultFontPointSize
+    font.family:    ScreenTools.normalFontFamily
+
     property bool _showHighlight: (pressed | hovered | checked)
-    QGCPalette { id: qgcPalDisabled; colorGroupEnabled: false }
-    background: Rectangle {
-        color:                  enabled ? (_showHighlight ? qgcPal.buttonHighlight : qgcPal.button) : qgcPalDisabled.button
+
+    QGCPalette { id: qgcPal; colorGroupEnabled: enabled }
+
+    contentItem: IconLabel {
+        spacing:    control.spacing
+        mirrored:   control.mirrored
+        display:    control.display
+        icon:       control.icon
+        text:       control.text
+        font:       control.font
+        color:      _showHighlight ? qgcPal.buttonHighlightText : qgcPal.buttonText
     }
-    contentItem: QGCLabel {
-        text:                   control.text
-        color:                  enabled ? (_showHighlight ? qgcPal.buttonHighlightText : qgcPal.buttonText) : qgcPalDisabled.buttonText
-        horizontalAlignment:    Text.AlignHCenter
-        verticalAlignment:      Text.AlignVCenter
-        elide:                  Text.ElideRight
+
+    background: Rectangle {
+        color: _showHighlight ? qgcPal.buttonHighlight : qgcPal.button
     }
 }