As it was using the `lines()` method of `Core::DeprecatedFile`, this
patch also introduce the usage of `BufferedFile` to take advantage of
its API: `can_read_line()` and `read_line()`.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
This unifies how 3DFileViewer handles the initial file when starting
the application and when opening files later on via the menu.
Errors are shown both for the initial load as well as when loading
files later on. An error during file load no longer clears the
existing model.
It also adds support for specifying the filename as a command-line
argument.
The opened file's name is shown in the titlebar.
