The ornamental design for a buttontool, as shown and described.
FIG. 1 is a perspective view of a button tool;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof, the right side being a mirror image thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line showing of a screw hole in FIGS. 1, 2 and 3 is included for the purpose of illustrating a portion of the button tool that forms no part of the claimed design.