Significantly improved SGUI

SGUI now has a window manager, and events are called in the proper
order. Higher up windows get events called before lower ones, and
visible focus changes take place when you click on a lower focused
window.
Fixed panel auto sizing

It was incorrect before. Also now it doesn't create vector objects every
time it works it out.
Small chatbox fix

When going over the message memory limit, a fairly large space would be
created at the top of the message list.
Merge pull request #37 from Person8880/develop

Develop