This function is made from what composed `set_file()` (which now calls
the new function). It allows to create a `HexDocumentFile` without
calling the hackish `set_file(move(m_file))`.
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 :^)
To support editing of large files it is an advantage to not load the
entire file into memory but only load whatever is needed for display at
the moment. To make it work, file access is abstracted into a socalled
HexDocument, of which there two: a memory based and a file based one.
The former can be used for newly created documents, the latter for file
based editing.
Hex documents now do track changes instead of the HexEditor. HexEditor
only sets new values. This frees HexEditor of some responsibility.
