Support for multiline tasks

This is an unofficial extension to todo.txt allowing to insert separators into the task which are displayed as newlines.
There is no change in the syntax of the task nor are there any additional newlines in the todo.txt file so it's still fully backwards compatible. It is only about interpreting the special sequence ' \\\\ ' (space-backslash-backslash-space) as a separator.

This is reflected in QTodoTxt in such a way that it now has a Multiline task edit dialog which automatically inserts the mentioned separator. This functionality can be enabled/disabled using the "Preferences" menu.

This features might somehow be related to issue #101