Removed usage of $.browser, improvements to selection handling code

$.browser has been deprecated for a while, and dropped from
jQuery 1.9. The code was using it to deal with browser differences
about text selection handling; references to $.browser have been
removed, and text selection handling has been enhanced to handle
all browsers (by using code from the jQuery.selection plugin).

More testing and tweaking may be required.
Moved dropdown close to the tag being typed

The tag list dropdown used to always appear at the bottom of the
text area; this is undesirable if the text area gets big and you
are editing a tag at the top of the text.

These changes move the dropdown close to the tag being edited
by the user.

This code is a bit kludgy and may require further tweaking, but
it is a good start.