[BL] Refactor random string generator.

Applications/FileIO/Legacy/createSurface.cpp

BaseLib/StringTools.cpp

BaseLib/StringTools.h

Tests/FileIO/TestBoostGmlInterface.cpp

Tests/FileIO/TestCsvReader.cpp

Tests/FileIO/TestCsvWriter.cpp

Tests/FileIO/TestTetGenInterface.cpp

Tests/FileIO_Qt/TestQtGmlInterface.cpp

Tests/GeoLib/IO/TestGLIReader.cpp

Tests/MeshLib/TestVtkMappedMeshSource.cpp