[FormLayout] Use even top and bottom spacing for separator

Summary:
The Kirigami separator has spacing below it, but not above it when in a Form Layout. This patch adds a check if there is a separator present and if it is, it adds spacing above it.

BUG: 405614

Test Plan:
Before:
{F6775026}

After:
{F6775027}

Reviewers: mart, #kirigami, ngraham

Reviewed By: mart, #kirigami, ngraham

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D20585

src/controls/FormLayout.qml

@@ -245,6 +245,7 @@ Item {
             visible: item.visible
             width: Kirigami.Units.smallSpacing
             height: Kirigami.Units.smallSpacing
+            Layout.topMargin: item.toString().indexOf("Separator")===0 ? Kirigami.Units.smallSpacing : 0 // FIXME: use item instanceof Kirigami.Separator when we can depend from Qt 5.11
             onItemChanged: {
                 if (!item) {
                     labelItem.destroy();