The ornamental design for a slideholder, as shown and described.
FIG. 1 is a perspective view of the design of the present disclosure;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a first side view thereof; and,
FIG. 7 is a second side view thereof.
Those portions of the present invention which are presented in phantom illustrate the environment of the design only and form no part of the claimed design. The dot-dash broken lines depict the bounds of the claim and form no part of the claimed design. Moreover, the shading presented in the accompanying drawings is provided merely to highlight the contour of the invention and is not intended to be illustrative of texture or gloss.