instead of error-prone tracking and updating offsets after each elementary change,
remember the initial children of each changed node, compare them with final ones
and fire events according to the difference

plus, cleanup and remove no longer needed API

related to WEB-29541

- insert default variable value or variable name, so that preview text make sense (and be highlighted correctly)
- test for the case emmet + vue

related to WEB-29541

OC-15517 PsiInvalidElementAccessException on extracting method to category, create from usage and other actions

In particular, this fixes the GitBranchContextTest

Now the detector detects only real conflicts, so the test should
prepare real conflicts, not just any merge commit.