SVGHMI: rework Jump widget to cope with Enable Expressions.

Relative jumps are not disabled implicitely anymore when pointed HMI_NODE is 0.
Also, "activable()" implementation was enhanced to make it optional.

<?xml version="1.0" encoding="UTF-8"?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>