Fix clicking on drop down button in wxAuiToolBar on wxMSW

The code handling the mouse events assumed the drop down button with was
10px, but wxAuiMSWToolBarArt uses a different width (14px). So clicking
on the left-most 4 pixels was not registered as a drop down click.

Allow the get (and set) the width of the drop down button of the
ToolBarArt.

Increase the detection area for drop down events, because the drop down
button is drawn 1 pixel larger than the actual size.

Rename some variables where dropdown was used instead of overflow.

Closes https://github.com/wxWidgets/wxWidgets/pull/939

interface/wx/aui/auibar.h

@@ -104,6 +104,13 @@ enum wxAuiToolBarArtSetting
     */
     wxAUI_TBART_OVERFLOW_SIZE = 2
 
+    /**
+      Drop down button size in wxAuiToolBar.
+
+      @since 3.1.2
+    */
+    wxAUI_TBART_DROPDOWN_SIZE = 3
+
 };
 
 /**