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 :^)
The FilePicker has implemented its common locations tray as a composite
widget built from a GUI::Frame with a bunch of GUI::Button inside it.
The problem with that is that it creates a long and annoying chain of
keyboard-focusable widgets.
This patch adds GUI::Tray, which is a dedicated single widget that
implements the same UI element, but without child widgets.
