Switched tree_list component to use global MODIFIER_KEY

What does this MR do and why?

This is a trivial refactoring of the tree_list component to use the global MODIFIER_KEY const (introduced in Migrate Markdown buttons to the Source Editor t... (!106951 - merged)) instead of the local one. This is the work in the course of Unify the definition of the modifier key on dif... (#385852 - closed)

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Related to #385852 (closed)