Add an option to make chat window always stay on top

requested here: http://www.igniterealtime.org/community/message/193342

Some users want to have an option to make a chat window always stay on top of other windows (dont know is it the same on all OSes). I see this like a button in a chat window toolbar, so a user would be able to quick and easily switch between "Always on top" and "Normal" modes. A toggle button. Such approach can be handy if separate windows feature would be added to Spark (option to have separate windows for every chat). Then every window could have a different setting. Though maybe this can be difficult to implement, so maybe a simple option in Preferences will do for now.