Merge branch 'breakspace'

This branch improves handling of spaces on display (see #10117):
* caret is correctly shown in the middle of double spaces in justified rows;

* sequence spaces are correctly shown at the end of rows before
  automatic row breaks.

Moreover, this branch:
* streamlines the code that handles spaces in row breaking

* improves display in Qt4 : although the improvements outlined above
  are not present in Qt4 for monospaced fonts, some dsplay glitches
  are resolved.

* improves performance for very long paragraphs (#12598).