Make MMB double click perform 'Zoom to Fit'

ADDED: Make middle mouse button double click do a 'Zoom to Fit'
in Eeschema, Pcbnew, and Gerbview.  Ctrl-MMB does a 'Zoom to Objects'
in Eeschema.

Fixes https://gitlab.com/kicad/code/kicad/issues/1988

eeschema/tools/ee_selection_tool.cpp

@@ -360,6 +360,15 @@ int EE_SELECTION_TOOL::Main( const TOOL_EVENT& aEvent )
                 m_toolMgr->RunAction( EE_ACTIONS::properties );
         }
 
+        // Middle double click?  Do zoom to fit or zoom to objects
+        else if( evt->IsDblClick( BUT_MIDDLE ) )
+        {
+            if( m_exclusive_or ) // Is CTRL key down?
+                m_toolMgr->RunAction( ACTIONS::zoomFitObjects, true );
+            else
+                m_toolMgr->RunAction( ACTIONS::zoomFitScreen, true );
+        }
+
         // drag with LMB? Select multiple objects (or at least draw a selection box) or drag them
         else if( evt->IsDrag( BUT_LEFT ) )
         {

gerbview/tools/gerbview_selection_tool.cpp

@@ -214,6 +214,12 @@ int GERBVIEW_SELECTION_TOOL::Main( const TOOL_EVENT& aEvent )
             m_menu.ShowContextMenu( m_selection );
         }
 
+        // Middle double click?  Do zoom to fit
+        else if( evt->IsDblClick( BUT_MIDDLE ) )
+        {
+            m_toolMgr->RunAction( ACTIONS::zoomFitScreen, true );
+        }
+
         else if( evt->IsCancel() || evt->Action() == TA_UNDO_REDO_PRE )
         {
             clearSelection();

pcbnew/tools/selection_tool.cpp

@@ -274,6 +274,12 @@ int SELECTION_TOOL::Main( const TOOL_EVENT& aEvent )
             }
         }
 
+        // Middle double click?  Do zoom to fit
+        else if( evt->IsDblClick( BUT_MIDDLE ) )
+        {
+            m_toolMgr->RunAction( ACTIONS::zoomFitScreen, true );
+        }
+
         // drag with LMB? Select multiple objects (or at least draw a selection box) or drag them
         else if( evt->IsDrag( BUT_LEFT ) )
         {