Fix bug introduced in previous commit : dual link list wasn't append and remove wasn't implemented correctly. Removed debug code, enhanced variable names and comments.

SVGHMI: optimization of C part : stop traversing the whole HMI tree, use dual linked list for subscriptions and single linked list for changes from HMI. Intermediate commit, still crashing in some cases.

Xenomai runtime: ensure that __DEBUG global variable is set when resuming debug.

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.

RUNTIME: Variable trace now uses limited list and buffer instead of flags in instance tree that was requiring systematical instance tree traversal, and worst size buffer. Forcing and retain still use tree traversal.

SVGHMI: more accurate filtering of elements to unlink or not

OPC-UA client: generated code now take UA_Variants on the stack directly, avoid useless allocation on write, and unalloc implicitely allocated data on read

OPC-UA client: generated code now take UA_Variants on the stack directly, avoid useless allocation on write, and unalloc implicitely allocated data on read