Add system button resources

This will be used in system buttons on flat dialog.
Add CBitmapButtonEx deprived from CBitmapButton

For making mouse hover bitmap buttons, I deprived from CBitmapButton and
add hover bitmap.
Add CFlatDialogEx deprived from CDialogEx

If deprived CFlatDialogEx, the dialog has white background, 3 system
buttons(min, max, exit), background drag moving and so on.