Add wxMac wxFileDialog::GetCurrentlySelectedFilterIndex() support

Simply set the relevant variable while changing selection and give
the extra control a chance to update.

Note that updating the UI currently asserts with the dialog sample if
wxOSX_FILEDIALOG_ALWAYS_SHOW_TYPES is 1 (toggable with Ctrl+Cmd+S in
the sample), This will be fixed in the next commit.

interface/wx/filedlg.h

@@ -236,10 +236,11 @@ public:
         control to update its state depending on the currently selected file
         type filter.
 
-        Currently this function is fully implemented only under MSW and
+        Currently this function is fully implemented under macOS and MSW and
         always returns @c wxNOT_FOUND elsewhere.
 
-        @since 3.1.3
+        @since 3.1.3 - MSW
+        @since 3.1.5 - macOS
 
         @return The 0-based index of the currently selected file type filter or
             wxNOT_FOUND if nothing is selected.

src/osx/cocoa/filedlg.mm

@@ -295,6 +295,10 @@ void wxFileDialog::DoOnFilterSelected(int index)
     NSArray* types = GetTypesFromExtension(m_filterExtensions[index],m_currentExtensions);
     NSSavePanel* panel = (NSSavePanel*) GetWXWindow();
     [panel setAllowedFileTypes:types];
+
+    m_currentlySelectedFilterIndex = index;
+
+    UpdateExtraControlUI();
 }
 
 // An item has been selected in the file filter wxChoice: