Make it possible to opt out of composing variables
Closes#6186
See merge request OpenMW/openmw!1076
(cherry picked from commit 15d278de554818fef6fecf300456800523e91adf)
4727ae4b Make it possible to opt out of composing variables
04e9b6d2 Abort on duplicate content file
->multitoken()->composing(),"settings where the values from the current source should replace those from lower-priority sources instead of being appended")