Handle multiple spaces at row break

Handle multiple spaces at row break

In order to work around the Qt row breaking algorithm, which considers
multiple spaces as one at QTextLine break, we insert word_joiner unicode
characters beteween each pair of spaces.

The TextLayoutHelper class makes it easy to handle that.

Update Row::Element::rtrim() to only remove one space at row end.

Update support::countExpanders() to count all spaces, without special
handling for consecutive ones.

Fixes bug #10117.