Some functions weren't obvious what they would do if an argument was
omitted. Split them up in to separate functions in those cases to
make the API clearer.
The panels were never meant to be positioned relative their container,
so make sure they pop up exactly where we want in the viewport, no
matter where they are in the structure.
This commit moves all the input-related files from `core/`
to `core/input/`, and renames a couple as relevant
(input.js --> input/devices.js, keyboard.js --> input/util.js).
The event-related wrapper functions in Util existed mainly for
backwards-compat. However, all currently supported browsers
support the standard functions, so these wrappers are no longer needed.
This commit restructures noVNC, splitting it into the core directory
and the app directory, with the former containing core noVNC parts,
and the latter containing parts specific to the application.
