implement line modification system

TODO:
- unit test
- improve visual appearance
- enable / disable in config page
- optionally show modified lines in scroll bar
- fine tuning

PS: Qt Creator's line modification system is totally buggy :^)
Example: http://wstaw.org/m/2011/09/03/plasma-desktopgz5185.jpg

CCBUG: 200753
CCMAIL: kwrite-devel@kde.org