Force application win_gravity to NorthWestGravity

LibreOffice incorrectly sets its top level windows win_gravity field
to StaticGravity. Resizing these windows to the left or upwards
results in a corrupted application with the application window moved
within the parent by the parent window resize.

Metacity forces window gravity for all application windows to
NorthWest, masking this application bug. xfwm4 should do the same.

A bug has been filed against LibreOffice as well.

Signed-off-by: Keith Packard <keithp@keithp.com>