This is a start in addressing issue PHPOffice#2965 (and earlier issue PHPOffice#749). Chart Titles are usually entered as strings or Rich Text strings, and PhpSpreadsheet supports that. They can also be entered as formulas (typically a pointer to a cell with the title text), and, not only did PhpSpreadsheet not support that, it threw an exception when reading a spreadsheet that did so.

This change does:
- eliminate the exception
- set a static chart title when it can determine it from the Xml

This change does not:
- fully support dynamic titles (e.g. if you change the contents of the source cell, or delete or insert cells or rows or columns)
- permit the user to set the title to a formula
- allow the use of formulas when writing a chart title to a spreadsheet
- provide styling for titles when it has read them as a formula