Fix various selection-related problems

Fix various selection-related problems

All these problems are related to what happens at the extreme points of rows

 * since VIRTUAL elements have a width but no contents, they have to
   be treated specially at some places. It would have been better to
   avoid testing for them explicitly, but I did not find a way.

 * Improve and cleanup the code in breakRow and fix in passing a crash
   when clicking on the right of an incomplete MARGIN_MANUAL
   paragraph.

 * improve the computation of row width in TextMetrics::computeRowMetrics.

 * handle properly the case where a position if not found on the row
   in both cursorX and getPosNearX (actually, this happens when
   selecting).

 * Some code cleanup and comments.