Implement Row::Element::row_flags

Implement Row::Element::row_flags

Move the enum definition RowFlags in its own include file, to avoid
loading Inset.h. Document it more thoroughly.

Rename RowAfter to AlwaysBreakAfter.

Add CanBreakInside (rows that can be themselves broken). This allow to
differentiate elements before bodyPos() and allows to remove a
parameter to shortenIfNeeded().

Make the Inset::rowFlags() method return int instead of RowFlags, as
should be done for all the bitwise flags. Remove the hand-made bitwise
operators.

Set R::E::row_flags when creating elements.
* INSET elements use the inset's rowFLags();
* virtual element forbid breaking before them, and inherit the *After
  flags from the previous element of the row;
* STRING elements usr CanBreakInside, except before bodyPos.

More stuff may be added later.