Setting correct width when changing artefact chooser tabs (Bug #1311876)

Currently the js wants to get the width of the <TD> element but it is
returning 0, getting the width from the <table> element returns
correct value.

Change-Id: I093cb56c0654ca2a90f521f6089b08cd7c227367
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>