LexemeHeaderWidget: way to address child components
Use $refs to address child components (in tests) - resolving open todo
and potential cause of flakyness, given that
"there’s no order guarantee for $children".
https://vuejs.org/v2/guide/components-edge-cases.html#Accessing-Child-Component-Instances-amp-Child-Elements
https://vuejs.org/v2/api/#vm-children
Bug: T195372
Change-Id: I54624467aaeb6d0576cd0eb09626e83afdee02d7