add tab drag handler (just repositions tab to track cursor on click+drag)
show placeholder as tab is moved; swap before/after positions
keep tabs inside tab container while dragging
fire sunk events when drag is completed
Merge pull request #197 from rstudio/feature/r-autocompletion-tweaks

Use source index to get completions from current package
pass mouseup events through to close button
use member prefs
call base onApply from editing preferences
updated R completion icons
updated function icon
update to pandoc 1.13.1 (on all platforms save for RHEL5)
update NEWS
change pandoc version to 1.13.1 on windows
check that completion from cache is not null in 'usingCache'
Merge pull request #199 from rstudio/bugfix/completion-cache-exception

check that completion from cache is not null in 'usingCache'
return empty completions rather than NULL
ensure 'methods' is actually loaded first
use .rs.normalizePath
eliminate 'mustWork' argument (unnecessary)
don't try an roxygen completion until an '@' has been inserted