Improve doc comments on existing Grid methods.
Refactor Grid class to call getRowCount().

Breaks the assumption that the number of rows in the Datasource is the
same as the number visible in the grid, which is necessary for access to
NEW rows that don't exist in the Datasource (yet).
Allow navigating to a NEW row of a Datasource.

Introduce a new event for a change in the number of rows accessible in
a Grid, independent of the underlying Datasource. For example, when
navigating to a NEW row that doesn't exist in the Datasource.
Use goog.base in com.qwirx.grid.Grid.prototype.enterDocument().

For Closure style.
Reduce code duplication in com.qwirx.grid.Grid.prototype.enterDocument().
Get row count from the event, for efficiency.

In com.qwirx.grid.Grid.prototype.onRowCountChange(), instead of asking the
data source again for its (new) number of rows.
Fix scrollbar maximum on an empty grid.