Desing better interface for table keywords #914

The TableElementFinder is poorly designed and it needs refactoring. The TableElementFinder does not support all the same locator strategies which are supported by the rest of library. It would be better to redesign the whole class to be better unified with the rest of the library.

Improvements for TableElementFinder

• Add more acceptance tests
• It should not matter which locator strategy is used to identify the table where actions are performed
• Support also webelements
• When identifying the table, support also identifying table by element above, example if DOM looks like this <div id="table"><table>...</table></div> then | Table Should Contain | id=table | item | should work
• Design is is possible to return table content to Robot side. It should be possible for simple tables like

<table>
  <tr>
    <th>Firstname</th><th>Lastname</th><th>Age</th>
  </tr>
  <tr>
    <td>Jill</td><td>Smith</td><td>50</td>
  </tr>
</table>

But with some complex table, example with nested cells and tables it might be really difficult. Also when table cell is empty, finding cells might be difficult.