fix bug in compact_path
catch errors when opening files given on command line
FlagLines: use a shared updaters for options

previously, having two windows showing the same buffer with
the same line flags would have updated the options twice, resulting
in wrong lines in option. Now line flags options are updated only
once by a shared object along all FlagLines highlighter using the
same option.
Fix UndoGroupOptimizer
add sentence (s) object
add paragraph (p) object