Hide outline until a document is selected

When no document was selected, the OutlineView's own "No sections"
empty-state label (count === 0 with an empty model) rendered centered
in the same pane as the "Select a document" placeholder. Both used
anchors.centerIn: parent, so the two messages overlapped and read as
crunched text. Hide the OutlineView until currentDoc is set so only the
"Select a document" placeholder shows.

ui/qml/Main.qml

@@ -242,6 +242,10 @@ ApplicationWindow {
                 anchors.fill: parent
                 anchors.margins: root.baseFontSize * 0.6
                 model: docModel
+                // Hidden until a document is selected so its own
+                // "No sections" empty state does not overlap the
+                // "Select a document" placeholder below.
+                visible: root.currentDoc !== ""
             }
 
             Label {