The ornamental design for a windowscreen, as shown and described.
FIG. 1 is a front perspective view of the window screen;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a top plan view thereof with the bottom view being a mirror image thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a side elevational view as seen from the left hand side of FIG. 4 with the right hand side elevational view being a mirror image thereof; and,
FIG. 6 is a rear elevational view thereof.
The claimed design is broken in the views to indicate indeterminate width and length.