basic autocomplete support - R side
add autocomplete to ace
modify ace autocomplete to allow asynchronous

although the underlying engine in ace autocomplete is asynchronous,
the callback mechanism is synchronous (probably because most people
don't like to think in CPS).  But we need async for calling out to R!
modifying ace r-mode to add completions

there might be a better way to customize this
this mod just plunks the rcloud callout directly into the mode,
sending the whole text and character index down, and using the
CPS of the mod to ext-language_tools.js
wrapper for rcloud.get_completions in ace autocomplete format

ace autocomplete requires a particular record format
this function takes the array of strings from r
and delivers those records
enable ace autocomplete in cell editors
artifacts
add autocomplete to input widget
hack in use of single selection

autocomplete seems to assume something about multiple selections
i.e. it uses getAllRanges() instead of getRange()
this doesn't seem to work for our use case
so just hack this in to get it working for the moment
artifacts