Update gitignore structure.

Refactor out gitignore in /other/serialTest.
Add some more noticable headers.
Some simplifaction and restructuring.
Clean up some files.
Refactor out lib platform gitignore files.
Refactor out some gitignore files from AdvancedImageLoading.
Remove erroneously committed XCode project components. Closes #2057.
gitignore: correct comment spacing, minor updates.
Merge pull request #2523 from bilderbuchi/gitignore-overhaul

Gitignore overhaul