Streamline Editor Architecture: Sheet Ordering, UI Lookup, and UnitUi Factory

[CG-Tespy]

What This PR Does

Moves responsibilities from FKMainEditor → FKEventSheet

• Determining the correct ordering of FKUnits for UI refresh
• FKEventSheet now exposes an ordered list of units, so FKMainEditor no longer reconstructs order manually in _populate_from_sheet().
• Writing FKUnits into the sheet
• Generating new FKEventSheet instances from a list of FKUnits
  This consolidates all sheet‑structure logic into the sheet itself, where it belongs.

Moves responsibilities from FKMainEditor → FKBlockContainerUi

• Gathering FKUnitUis and their represented FKUnits
• FKBlockContainerUi now maintains a lookup table and exposes units / unit_uis properties, so clients no longer need to manually iterate children or reconstruct ordering.
  This makes the container the authoritative source for UI‑side block order and lookup.

Introduces a new class: FKUnitUiFactory

• Responsible for creating FKUnitUi nodes from FKUnits
• Currently supports EventRows, Comments, and Groups
• (Condition/action UIs will be added later once structural issues are resolved)
  This removes UI instantiation logic from FKMainEditor and centralizes it in a dedicated factory.

Small Improvements

• FKUnitUi’s before_block_changed and block_changed signals now explicitly specify that the passed node is an FKUnitUi.
• Added optional debug print logs to aid troubleshooting.

Bugfix(es)

• Fixed a regression that prevented comment reordering from persisting.