Fix str() + Content()

[gvx]

Content.__radd__ does not currently behave as expected. For example:

>>> from textual.content import Content
>>> Content('foo') + 'bar'  # works as expected
Content('foobar')
>>> 'foo' + Content('bar') # expected: Content('foobar')
Content('barfoo')

The problem is that Content.__radd__ (which implements other + self) returns self + other, which is incorrect for concatenation. This pull requests fixes that and adds tests for it. (Existing tests only test for the empty string, for which a + b == b + a happens to hold, unlike most strings.)

[willmcgugan]

thanks