As suggested by Joshua, this commit adds the 2-clause BSD license as a
comment block to the top of every source file.
For the first pass, I've just added myself for simplicity. I encourage
everyone to add themselves as copyright holders of any file they've
added or modified in some significant way. If I've added myself in
error somewhere, feel free to replace it with the appropriate copyright
holder instead.
Going forward, all new source files should include a license header.
Since we're moving to a world of ref-counting, we can't have weird
behaviors like "windows delete themselves when you close them."
The "close app when there are no more windows" mechanism is moved
to GWindow::hide(). Now, we close the app when it has no more
windows on screen.
