Rename InsetMathHull::isTable to allowsTabularFeatures

Rename InsetMathHull::isTable to allowsTabularFeatures

The old name conflicted with the newly introduced Inset::isTable.

Now the meaning is as follows.
* Inset::isTable() is true when the inset is composed of lines and columns
* InsetMathHull::allowsTabularFeatures is true when the current type of hull allows for tabular-like functions.

(cherry picked from commit db39dc7ccd4c2dd14ef20547e6e3c056ba030d83)