In 7c5e30daaa, the focus was "only" on
Userland/Libraries/, whereas this commit cleans up the remaining
headers in the repo, and any new badly-formatted include.
Selecting an Outline Item from the Outline view informs via callback the
corresponding Destination that has been selected. This will be used to
move the application to the corresponding page/section/etc.
This follows the same idea that Andreas was doing in this latest videos,
where construction-time TRY()s were not present but should have been.
Like Andreas did, moving the initialisation of such fields to the
factory function, which then returns ErrorOr solves the issue.
