Implement support for column and cell content assertion.

Considering the following grid:
1 -> |a|b|c|
2 -> |a|d|c|
3 -> |b|a|c|
I would like to assert that the value of the second cell in the third row is a
The improved API is needed for ab-testing project.

Suggestion for API extension:
for ContentApp

boolean hasGridColumn(String columnName)
RowPageObject selectRowByPath(rowPath)

for RowPageObject

  String getCellContentByColumnName(String columnName)