Improve handling of LOBs in result view (Oracle)

When a query results contains LOBs and the number of results exceeds the visible area (so that the result view has scrollbars) an error is logged on console:

Traceback (most recent call last):
  File "/home/andi/devel/runsqlrun/rsr/worksheet/results.py", line 331, in do_get_value
    data = str(value)
cx_Oracle.ProgrammingError: LOB variable no longer valid after subsequent fetch

LOBs shouldn't be accessed directly when displayed in result list. Instead a simplified text version should be used.

In addition: resizing a column containing LOBs is verrry slow.