* FileDialog.cs: Accept any FilterIndex value, and store it

* FileDialog.cs: Accept any FilterIndex value, and store it
unmodified. When FilterIndex is less than 1, or greater than number
of filters, then default to first filter. Only add filter extension to
file if user did not specifiy an extension. When type of dialog is
OpenFileDialog and DefaultExt is set, then only use filter extension
if: CheckFileExists is true and no file wih the default extension
exists, or CheckFileExists is false, and user specified file does not
exist. When CheckFileExists is true, then add first extension of
selected filter that matches existing file. Perform checks for
existing file, overwrite and create after extension has been added to
file name. When CheckFileExists is true and type is SaveFileDialog,
then only consider first filter extension if DefaultExt is set.
When CheckFileExists is true, then ignore DefaultExt if file with that
extension does not exist. Also perform check for existing file when
type is SaveFileDialog. Changed some field to constants.

svn path=/trunk/mcs/; revision=82935