Enable drag to resize function browser, data selector

Put the "additional widgets" in a QSplitter so their relative sizes can be
adjusted. This is useful when a complicated composite function with many
parameters is used, so that more can be seen without needing to scroll the
function browser.

Use stretch factor to ensure that the additional widgets get any extra space
when the window is resized.

re #15518

MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/MuonFitPropertyBrowser.h

@@ -16,6 +16,7 @@ class QtEnumPropertyManager;
 class QtProperty;
 class QtBrowserItem;
 class QVBoxLayout;
+class QSplitter;
 
 namespace Mantid {
 namespace API {
@@ -112,6 +113,8 @@ private:
                                   const int nWorkspaces) const;
   /// Layout for extra widgets
   QVBoxLayout *m_additionalLayout;
+  /// Splitter for additional widgets
+  QSplitter *m_widgetSplitter;
   /// Names of workspaces to fit
   std::vector<std::string> m_workspacesToFit;
   /// Label to use for simultaneous fits

MantidQt/MantidWidgets/src/MuonFitPropertyBrowser.cpp

@@ -41,6 +41,7 @@
 #include <QMessageBox>
 #include <QAction>
 #include <QLayout>
+#include <QSplitter>
 
 namespace {
 Mantid::Kernel::Logger g_log("MuonFitPropertyBrowser");
@@ -149,8 +150,14 @@ void MuonFitPropertyBrowser::init() {
   auto parentLayout = qobject_cast<QVBoxLayout *>(w->layout());
   if (parentLayout) {
     const int index = parentLayout->count() - 2;
-    parentLayout->insertLayout(index, m_additionalLayout);
+    constexpr int stretchFactor = 10; // so these widgets get any extra space
+    parentLayout->insertLayout(index, m_additionalLayout, stretchFactor);
   }
+  m_widgetSplitter = new QSplitter(w);
+  m_widgetSplitter->setOrientation(Qt::Vertical);
+  m_widgetSplitter->setSizePolicy(QSizePolicy::Policy::Expanding,
+                                  QSizePolicy::Policy::Expanding);
+  m_additionalLayout->addWidget(m_widgetSplitter);
 }
 
 /**
@@ -469,8 +476,10 @@ void MuonFitPropertyBrowser::finishAfterSimultaneousFit(
  * @param widget :: [input] Pointer to widget to add
  */
 void MuonFitPropertyBrowser::addExtraWidget(QWidget *widget) {
-  if (m_additionalLayout) {
-    m_additionalLayout->addWidget(widget);
+  widget->setSizePolicy(QSizePolicy::Policy::Expanding,
+                        QSizePolicy::Policy::Expanding);
+  if (m_additionalLayout && m_widgetSplitter) {
+    m_widgetSplitter->addWidget(widget);
   }
 }