Add VARIABLE_SET node type, variable-related messages (VariableSetMode,
VariableSetID, VariableDataValues, VariableScope, VariableDataMap, etc.)
to the Kiwi schema.
Export: emit VARIABLE_SET and VARIABLE nodeChanges on an internal-only
canvas page, include variableConsumptionMap on nodes with boundVariables.
Import: rewrite variable import to use variableSetModes, variableSetID,
variableResolvedType, and variableDataValues (multi-mode COLOR/FLOAT/
BOOLEAN/STRING/ALIAS). Import variableConsumptionMap bindings.
Closes#65
d9fbc6fb92