Add method to TreeItem that allows setting text to a cell

Currently, there is no way how to set a text into a TreeItem's cell.
e.g., set a text to the cell in column Value:
![tree](https://cloud.githubusercontent.com/assets/4416361/2616117/71d18fde-bc02-11e3-8be7-3741cd4bfdf3.jpg)
[source: https://github.com/jbosstools/jbosstools-webservices/blob/4edef2633be20bd88617401c47b224864dfec0e9/plugins/org.jboss.tools.ws.jaxrs.core/src/org/jboss/tools/ws/jaxrs/core/internal/metamodel/builder/JaxrsElementChangedProcessorDelegate.java]

In SWTBot, there is a workaround for this problem. SWTBotTreeItem's method click(cellIndex) gains focus to the specified cell, which activates Text widget.

SWTBotTreeItem.click(1); //gain focus to the cell with index 1
bot.text().setText("...");