Add new row flags Flush and FlushBefore to let insets indicate whether
they cause flushing of current row (eg. newline) or of previous row
(e.g. display insets).
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.
