QmlDesigner: Fix the position of placeholder in FilterComboBox

The AutoCompleteString view had a bad position. The text property of the
TextMetrics was wrong, which caused the placeholder to have bad position
since the position was calculated with wrong text.

In addition, I prevented showing the placeholder when the root comboBox
is not opened, because otherwise it may show text when no item is
selected.

The main task was QDS-7662, which is now divided.

Task-number: QDS-7912
Change-Id: Ib937d923191ec3c544dae3259774cd2bb5b7adb0
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

share/qtcreator/qmldesigner/propertyEditorQmlSources/imports/StudioControls/FilterComboBox.qml

@@ -547,20 +547,27 @@ Item {
             ]
 
             Text {
-                visible: root.autocompleteString !== ""
+                id: tmpSelectionName
+                visible: root.autocompleteString !== "" && root.open
                 text: root.autocompleteString
                 x: textInput.leftPadding + textMetrics.advanceWidth
-                y: (textInput.height - Math.ceil(textMetrics.height)) / 2
+                y: (textInput.height - Math.ceil(tmpSelectionTextMetrics.height)) / 2
                 color: "gray" // TODO proper color value
                 font: textInput.font
                 renderType: textInput.renderType
-            }
 
                 TextMetrics {
                     id: textMetrics
                     font: textInput.font
                     text: textInput.text
                 }
+                TextMetrics {
+                    id: tmpSelectionTextMetrics
+                    font: tmpSelectionName.font
+                    text: "Xq"
+                }
+            }
+
 
             Rectangle {
                 id: checkIndicator