SVGHMI: Adapt svghmi.c to changes in UnpackVar

Runtime: Now retain uses a list based on data available in VARIABLES.csv instead of traversing the whole instance tree looking for RETAIN flag.

RUNTIME: 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. Retain still uses tree traversal.