core cleanup: move plugin generation to purple

core cleanup: move plugin generation to purple

The first real step in the core cleanup. Build result in core is now a linker
library that gets linked by the plugin code. For now only one line of code has
been moved from core to purple.

Introduced a temporary header file for the core de-purple transition period.
This should only be included by sipe.c (in core) and the purple code to make
sure that both sides use the same function prototypes.

Tests moved to purple. They can be moved back to core once we figure out how
to de-purple-ize the code.

NOTE: Windows build is *definitely* broken by this :-)