Invalidation Performance, Rename LineStorage index to offset

[thecoolwinter]

Description

Fixes a few small things:

• No longer nukes layout information on every invalidation call. Instead tells the layout view that it needs layout, meaning layout will be performed when the system determines it's necessary.
  • When doing something like syntax highlighting, this means that layout will be calculated once for each invalidated line, rather than once per color update.
• When editing text, forces a layout pass to ensure edited lines are updated.
• Renames a confusing atIndex to atOffset to avoid potential confusion in the TextLineStorage object.
• Fix a small missing notification and delegate update in setTextStorage.
• Moves layout manager layout methods to their own file.

Related Issues

N/A, general cleanup.

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

N/A