loongoffice/filter

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 (!).