Support displaying correct difference for multi-line strings

[vbreuss]

Currently two methods highlight the difference between two strings:

• StringEqualsExpectedValueAssertCondition
• ThrowsWithMessageEqualToExpectedValueAssertCondition

There is a bug, that the index of first difference for multiline-strings is incorrectly set to 0, when the first difference is on the first character of a new line, e.g.

foo
bar

and

foo
something-other

Also moved the calculation and display of such a difference to a common helper class.

• 764