Fix chat scroll bar widget jumping

Hovering over the chat window's scroll bar was causing it to jump
around. Replaced the 12px style spacing that was causing the problem
with a widget-based solution.
Fix chat window scrolling to stay at bottom

Stay at bottom window when:
- There is new chat.
- People come and go from online list.
- Chat window hides and restores when application focus changes.
- Interface is minimized and restored.
Merge pull request #2926 from ctrlaltdavid/chat_window

Fix chat window scrolling