Expose application browser model instance to iface

Reusing the existing browser model is more efficient then reconstructing
new models, so it's useful for plugins to have access to this instance
too.

python/gui/auto_generated/qgisinterface.sip.in

@@ -1142,6 +1142,14 @@ is chosen. Dialog is shown only if global option is set accordingly.
 :return: true if a datum transform has been specifically chosen by user or only one is available.
 
 .. versionadded:: 3.0
+%End
+
+    virtual QgsBrowserModel *browserModel() = 0;
+%Docstring
+Returns the application browser model. Using this shared model is more efficient than
+creating a new browser model for every use.
+
+.. versionadded:: 3.4
 %End
 
   signals:

src/app/qgisappinterface.cpp

@@ -792,5 +792,10 @@ bool QgisAppInterface::askForDatumTransform( QgsCoordinateReferenceSystem source
 
 void QgisAppInterface::takeAppScreenShots( const QString &saveDirectory, const int categories )
 {
-  return qgis->takeAppScreenShots( saveDirectory, categories );
+  qgis->takeAppScreenShots( saveDirectory, categories );
+}
+
+QgsBrowserModel *QgisAppInterface::browserModel()
+{
+  return qgis->mBrowserModel;
 }

src/app/qgisappinterface.h

@@ -553,6 +553,7 @@ class APP_EXPORT QgisAppInterface : public QgisInterface
 
     void takeAppScreenShots( const QString &saveDirectory, const int categories = 0 ) override;
 
+    QgsBrowserModel *browserModel() override;
 
   private slots:
 

src/gui/qgisinterface.h

@@ -48,6 +48,7 @@ class QgsOptionsWidgetFactory;
 class QgsLocatorFilter;
 class QgsStatusBar;
 class QgsMeshLayer;
+class QgsBrowserModel;
 
 #include <QObject>
 #include <QFont>
@@ -937,6 +938,13 @@ class GUI_EXPORT QgisInterface : public QObject
       */
     virtual bool askForDatumTransform( QgsCoordinateReferenceSystem sourceCrs, QgsCoordinateReferenceSystem destinationCrs ) = 0;
 
+    /**
+     * Returns the application browser model. Using this shared model is more efficient than
+     * creating a new browser model for every use.
+     * \since QGIS 3.4
+     */
+    virtual QgsBrowserModel *browserModel() = 0;
+
   signals:
 
     /**