The ornamental design for a touchpad, as shown and described as shown and described.
1. Touchpad
1.1 is a side elevation view.
1.2 is an opposite side elevation view, shown in use with a disclaimed keyboard.
1.3 is a front elevation view.
1.4 is a top plan view.
1.5 is a front and top perspective view, shown in use with a disclaimed keyboard.
1.6 is rear and side perspective view, shown in use with a disclaimed keyboard.
The white keyboard surrounded with broken lines shown is included for the purpose of illustrating environment of the touchpad and forms no part of the claimed design.