The ornamental design for a tablebase, as shown and described.
FIG. 1 is a front elevational view of the table base, a rear elevational view, a left side elevational view, and a right side elevational view thereof being identical;
FIG. 2 is a top plan view thereof; and,
FIG. 3 is a bottom plan view thereof.
The broken lines in the figures are for illustrative purposes only and form no part of the claimed design.