Enable adding widgets to MuonFitPropertyBrowser

Add an extra (empty) layout to MuonFitPropertyBrowser.
Expose this layout so that extra widgets can be placed there.
The widgets go after the buttons and tip, but before the browser.

Use case: want to put MuonFitDataSelector and FunctionBrowser there

re #15518

MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/MuonFitPropertyBrowser.h

@@ -14,6 +14,7 @@ class QtStringPropertyManager;
 class QtEnumPropertyManager;
 class QtProperty;
 class QtBrowserItem;
+class QVBoxLayout;
 
 namespace Mantid
 {
@@ -44,6 +45,8 @@ public:
   void setWorkspaceName(const QString &wsName) override;
   /// Called when the fit is finished
   void finishHandle(const Mantid::API::IAlgorithm *alg) override;
+  /// Add an extra widget into the browser
+  void addExtraWidget(QWidget *widget);
 
 public slots:
   /// Perform the fit algorithm
@@ -66,6 +69,8 @@ private:
   void populateFunctionNames() override;
   /// Check if the workspace can be used in the fit
   bool isWorkspaceValid(Mantid::API::Workspace_sptr) const override;
+  /// Layout for extra widgets
+  QVBoxLayout *m_additionalLayout;
 };
 
 } // MantidQt

MantidQt/MantidWidgets/src/MuonFitPropertyBrowser.cpp

@@ -38,6 +38,7 @@
 #include <QSettings>
 #include <QMessageBox>
 #include <QAction>
+#include <QLayout>
 
 namespace MantidQt {
 namespace MantidWidgets {
@@ -132,6 +133,15 @@ void MuonFitPropertyBrowser::init() {
 
   // Initialise the layout.
   initLayout(w);
+
+  // Create an empty layout that can hold extra widgets
+  // and add it after the buttons but before the browser
+  m_additionalLayout = new QVBoxLayout();
+  auto parentLayout = qobject_cast<QVBoxLayout *>(w->layout());
+  if (parentLayout) {
+    const int index = parentLayout->count() - 2;
+    parentLayout->insertLayout(index, m_additionalLayout);
+  }
 }
 
 /**
@@ -335,5 +345,15 @@ void MuonFitPropertyBrowser::finishHandle(const IAlgorithm *alg) {
   FitPropertyBrowser::finishHandle(alg);
 }
 
+/**
+ * Adds an extra widget in between the fit buttons and the browser
+ * @param widget :: [input] Pointer to widget to add
+ */
+void MuonFitPropertyBrowser::addExtraWidget(QWidget *widget) {
+  if (m_additionalLayout) {
+    m_additionalLayout->addWidget(widget);
+  }
+}
+
 } // MantidQt
 } // API