Add tooltips to surface form cells

[jcrippen]

It might be possible for a PDF to include tooltips on arbitrary non-hyperlink text. If this can be done then we could add tooltips to the surface form entries in each cell. The tooltip could contain the complete underlying prefix string that corresponds to the surface form. For example, table 85 “N-conjugation n- with irrealis u- and modal g̱-, first person singular subject x̱-” has a cell containing oonḵwadzi. This corresponds to the prefix string a-u-n-g̱-x̱-d-s-i-, from the horizontal a-u-n-g̱-x̱- and the vertical d-s-i-. If a reader using an app like Acrobat or macOS Preview hovers the mouse over the form they would see a popup tooltip containing the prefix string. Since this information would be encoded in the LaTeX source it would also be programmatically extractable and hence would simplify developing an automated mapping between prefix strings and surface forms.