Allow breaking a row element at a leading/trailing space

Allow breaking a row element at a leading/trailing space

When an inset is separated from the adjacent string by a space, it is
reasonable to be able to break the string after the space.
Unfortunately, QTextLayout does not do that.

This patch reverts the workaround inserted in 71378268 and replaces it
with a different trick: the string is enlosed between a pair of
zero-width non breaking space characters, so that the leading/trailing
spaces are now normal spaces, where QTextLayout will agree to break
the string.

    Fixes bug #9921 for good.