[gephi-dev] Export API Refactoring

[Mathieu Bastian]

The Export API has been refactored and is now considered as a stable
API. After thinking about uses cases and doing ImportAPI refactoring
the Export features became more clear and a good refactoring could be
done.

The API now supports new use-case, including:
- Export as a Writer or as a Stream. One could export to a String
using StringWriter easily.
- Exporters that are not graph or vector graphics file could be done
in the SPI and integrated in the system.
- Custom Exporter could be directly used from the controller

Tasks done:
- Create an ExporterBuilder interface for exporter creation and
different exporters: GraphExporter, VectorExporter, ByteExporter and
CharacterExporter that covers common cases.
- The way exporters write data has been rationalized by using either
java.io.Writer (text) or java.io.OutputStream (byte).
- The ExportController has been improved to support all use-cases,
including file, writer and stream export.

The ExportPluginUI is a new module that hosts the Export UI, including
database (moved from DesktopExport).