Added wxFLP_SMALL and wxDIRP_SMALL styles for wx{File,Dir}PickerCtrl.

Added wxFLP_SMALL and wxDIRP_SMALL styles for wx{File,Dir}PickerCtrl.

These styles allow to use a smaller browse button as the standard one takes
too much space, often leaving too little of it for the more important text
control part.

Notice that both styles are, in fact, equal to wxPB_SMALL but only file and
directory pickers currently use it as it doesn't make sense for the colour and
font pickers.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68921 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775