Fix missing part library broken by schematic I/O plugin changes.
The LIB_PART object stores a pointer to the PART_LIB that it belongs to.
Now that the PART_LIB is no long responsible for loading the library, add
the PART_LIB pointer after the library is loaded by the plugin.
Prevent schematic I/O plugin from setting the library cache to null.
Don't call init() when performing library functions as it was always
setting the m_cache variable to null which cause the library to be
reloaded every time a library command was performed as well as a
memory leak.
Remove library caching from PART_LIB and use caching provided by SCH_PLUGIN.
Add support to use PROPERTIES for buffering and document file write control
instead of adding code the SCH_PLUGIN object in the SCH_LEGACY_PLUGIN that
will not be required when the new file formats are implemented.
Add buffering to SCH_LEGACY_PLUGIN to prevent cache from writing file on
every change to library. This is to prevent the cache library from being
written every time a new symbol is added.
Add option to not save library document file when saving library. This is
primarily used by the cache library write code.
Move symbol library write code out of LIB_PART and into SCH_LEGACY_PLUGIN.
Add exception handling where LIB_PART caught the exception and returned
an error status.
Remove KICAD_SCH_IO_MANAGER build option as it is no longer optional.
