Delimited text bug fixes 2 - addresses bug #7688

This patch addresses issue http://hub.qgis.org/issues/7688 - QgsFeatureRequest filter type FilterFid not being respected in delimited text provider.  

Also addresses a potential core dump from QgsVectorLayerFeatureIterator if invalid attributes requested from in QgsFeatureRequest.setAttributesSubset, identified by new test cases for delimited text provider.

Contains:
Fix to QgsDelimitedTextFeatureIterator
Fix to QgsVectorLayerFeatureIterator
Test cases for QgsFeatureRequest options on delimited text provider.

Also fixed problem with very long text fields (eg wkt files) corrupting preview table in GUI.  Fields in preview are abbreviated if very long.  Similarly limited the length of field names (somewhat arbitrarily).

Also added usability fix - if all records are badly formatted then dialog box may show "No data in file".  Can be confusing for users (was for me and I wrote that bit!?).  So have added message informing that records are discarded
because of bad formatting.

Also fixed some log messages going to wrong log (ie not delimited text)