Fix visible usage

Change in visibility was causing combo box contents to reset. Visible
goes to false even when you move away from Plan view, not just when the
combo is unused.

src/FirmwarePlugin/APM/CopterGeoFenceEditor.qml

@@ -43,6 +43,8 @@ Column {
             width:  editorColumn.width
             height: textField.height
 
+            property bool showCombo: modelData.enumStrings.length > 0
+
             QGCLabel {
                 id:                 textFieldLabel
                 anchors.baseline:   textField.baseline
@@ -55,7 +57,7 @@ Column {
                 width:          _editFieldWidth
                 showUnits:      true
                 fact:           modelData
-                visible:        !comboField.visible
+                visible:        !parent.showCombo
             }
 
             FactComboBox {
@@ -63,8 +65,8 @@ Column {
                 anchors.right:  parent.right
                 width:          _editFieldWidth
                 indexModel:     false
-                fact:           visible ? modelData : _nullFact
-                visible:        modelData.enumStrings.length
+                fact:           showCombo ? modelData : _nullFact
+                visible:        parent.showCombo
 
                 property var _nullFact: Fact { }
             }