substitute variables in custom sheets
start refactoring variable replacement
start testing variable_substituter
simplify unit test for callerpicture substitution
add substituter.substitute_with_default
change tests for resolv_line_content

tests are now in tools/tests/test_variable_substituter.py
implement sheet.variable_values
use sheet.variable_values for custom sheets
clean a bit the reading of sheet UI file
look for variables in VariableAggregator for sheets
Merge remote-tracking branch 'origin/4778-variable-substitution-in-custom-sheets-3'