Filter registration and some simple filters (also descriptions).

Desperate splitting of code into small shared libraries for historical
reasons presumably (OS/2 and Windows 3.x). The libraries produced from
the code in each subdirectory of filter/source/graphicfilter are
graphic format import or export filters. But they don't have uniform
API. Some have either an GraphicImport or GraphicExport entry point,
and are loaded and used in a uniform fashion from code in
svtools/source/filter/filter.cxx. Others have different API and are
loaded from other places. For instance "icgm" has ImportCGM, and is
loaded and used by sd/source/filter/cgm/sdcgmfilter.cxx (!).
Svgreader is used for "File->Open" and then to choose the svg file.
For "Insert->Picture->From File", see svgio/source/svgreader directory.

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filter configuration
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The filter configuration consists of two parts, the type definition in
filter/source/config/fragments/types/ and the actual filter definition
in filter/source/config/fragments/filters/.

Each file type e.g. text file should be represented by exactly one
type definition. This type can then be referenced by several different
filters, e.g. calc text, writer text.