[Enhancement] pin icon instead of square icon

[Superchicken]

As a new user I accidentally pinned tabbychat by clicking the pin button which is just a square, relevant #28. The resulting behavior is fairly frustrating if you are not aware of pinned mode or how to revert it. (Yes I am that stupid)

A pin icon instead of a square button would make it fairly obvious as to what the button does. It would help new users out quite a bit while learning the features of tabbychat.

Something similar to http://www.fileformat.info/info/unicode/char/1f4cd/index.htm or http://www.fileformat.info/info/unicode/char/1f4cc/index.htm would be nice.