'Insert parens' option + respect insert matching

This adds a UI pref controlling whether we insert parentheses following a function completion, and also ensures that the insertion behaviour respects the 'insert matching' pref for R and C++ modes.
You're right, it interferes with the display of the tooltip -- it gets displayed initially, but disappears immediately after the next keystroke (e.g. just pressing SHIFT alone removes it, as one might do when preparing to insert a parenthesis manually)