GridSheet should handle column and row tab labels

[dkrasner]

We should be able to reference columns and rows, and coordinates in general, using tab label. For example, GridSheet should methods like

.putAt(c, value, notify){
      pointOrCoord = this.getPointOrCoordFromRef(c);
      this.dataFrame(pointOrCoord, value, notify);
}

where c is either Point, [x, y], "x,y" or a label "ColtabRowtab like "A1" (== [0, 0] coordinate).

We could distinguish based on type and string structure.

At the moment this happens up in Worksheet which doesn't make sense and could lead to mismatch issues down the line.

NOTE: this can't work using DOM queries for tab element attributes, because we might be referencing elements which are not in view.

Similarly we need to think about the notion of column names at some point (or maybe not...)