#69 samples\wx\PropertyGrid.hs: date changes format

[Henk-Jan van Tuyl]

The date in sample program PropertyGrid.hs is displayed in a wrong format (mm/dd/yyyy); when the date field is selected, the format changes to dd-mm-yyyy, which corresponds to the locale. When another field is selected, the format changes back to mm/dd/yyyy.

OS: Windows XP
GHC version: 7.6.3
wxWidgets version: 2.9.5
wxHaskell version: 0.90.1.0 (currently in Github repository)