Fix BaseTextmark lineNumber tracking

The following sequence would add the BaseTextmark to the wrong line:
1) Add a mark on line x
2) Move line x down by addign newlines above
3) Close and reopen the file
=> Mark would be readded to line x

Change-Id: Ia5d580b5893331974fb908e367b74df69fbb6572
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>