Rewrite the Path tool's transaction management logic in delete messages

https://github.com/GraphiteEditor/Graphite/pull/3111 attempts to fix the broken transactions and undo history when performing delete operations in the path tool. The issue is resolved but this is not the ideal way of implementing it correctly (currently it is passing flags between different methods). This should be re written.

Review PR #3111 and the current Path tool delete-message transaction flow first. Identify how transaction and undo-history handling is split across methods; done means delete operations retain correct transaction and undo behavior without the current flag-passing approach.

索引モデルが issue の本文から書いたものです。