add Column attribute for help tooltip in heading

To use: add attribute help_text=_("foo") to tables.Column(...)
This will add a help icon next to the Column name which
you can hover-over to get informational text.

Change-Id: I10a28bd51efbaf83c617b30b31d861445418f9d8
Closes-Bug: #1381633
Merge "add Column attribute for help tooltip in heading"