fix #26 make autoscroll disable work

Autoscroll is disabled by scrolling up scrolling down to
the end, re enables it.

Tested on Linux only.
A comment states that GetScrollThumb() does not work on Windows,
if it is still true, it might not work there.

Based on 6cba30bed7b4dbd3f177403d1bbf97180e7e02f9
original code didn't reenable redraw by calling
m_chatlog_text->Thaw();

There is some jiggling and flickering sometimes,
I think because the display position is determined by the line-text.
I have tested by bashing at the keyboard and producing similar lines
Merge pull request #46 from baracoder/fix_26

fix #26 make autoscroll disable work