Add Truncate() method to plStringStream, which truncates the buffer without reallocating memory
Use plString in pfLocalizationMgr.

Conflicts:
	Sources/Plasma/FeatureLib/pfLocalizationMgr/pfLocalizationDataMgr.cpp
	Sources/Plasma/FeatureLib/pfLocalizationMgr/pfLocalizationDataMgr.h
	Sources/Plasma/FeatureLib/pfLocalizationMgr/pfLocalizedString.h
Use plString in pfLocalizationMgr - Part 2

More clean-up and adjustments to use plString in pfLocalizationMgr
and associated classes.
Convert plLocalizationEditor to use plString.

Required to match previous commits to pfLocalizationMgr and retain functionality.
Condense string conversion in Python glue.

Adds fixes from code review.
Fix substring indexing and whitespace.

Fixes an off-by-one and off-by-initial-offset error,
not clearing the stream between loops,
and a few other edge conditions.