show a file dialog

def call(self):
        '''show a file dialog'''
        from wx_loader import wx

        # remap flags to wx descriptors
        flag_map = {
            'open': wx.FD_OPEN,
            'save': wx.FD_SAVE,
            'overwrite_prompt': wx.FD_OVERWRITE_PROMPT,
        }
        flags = map(lambda x: flag_map[x], self.flags)

        dlg = wx.FileDialog(None, self.title, '', "", self.wildcard, flags)
        if dlg.ShowModal() != wx.ID_OK:
            return None
        return dlg.GetPath()