Rewrite word split for autocomplete, fix #930 fix #681

Rewrote the word split in a simple imperative way, a regex solution wouldn't have been simple and neither was the current option.

Changes behavior by splitting the string by actual whitespace boundaries and not by regex's definition of word which was lacking, in practice this means:

pros:

• @lain@lain.c is now considered a single word so autocomplete works for instance too (helps when someone has lots and lots of alts
• fixes #930 (closed)

cons:

• if you write @lain, then move cursor before the comma, it won't open autocomplete for @lain if you don't add a space. (while logically , shouldn't be considered a part of the word, other similar characters like . or - should not be taken out, so I kept it simple)