Text input field auto-resize for chat window

Chat window's input field now follows its internal text size. One can disable
this via "Format->Input auto-resize" and got back to the standard behaviour.
For technical reasons, manual resizing doesn't work properly with auto-resizing
enabled - size is reset to standard on every input.

This patch comes from old svn reviewboard request 7065 and was not commited to
old svn repository because of svn to git migration.

REVIEW: 7065
CC