Added RETAIN qualifier to VARIABLES.CSV

Variable forcing now uses limited list and buffer instead of systematical instance tree traversal and in-tree "fvalue" to keep track of forced value for pointed variables (external, located). Pointer swapping is performed when forcing externals and located, with backup being restored when forcing is reset. Match Beremiz commit 93ad018fb602.

Fixed __SET_EXTERNAL_FB declaration in accessor.h, invoked when accessing EXTERNAL_BLOCK.VARIABLE. Macro was invoking __SET_VAR with wrong parameter count. Fixed corresponding code generator for ST. Seems similar codegen for IL is also broken, but fix would affect other (unknown) behviours -> didn't touch it.