Having to rely on GUI::Desktop's on_rect_change is quite limiting and a
bit awkward (continuing to use it would mean having to setup the
callback in every application using a webview) - we need a better way of
letting widgets know of a screen rect change automatically.
The specific use case will be IPWV/OOPWV which need to keep track of the
screen rect and notify the WebContent service of any change (which on
its own deliberately can't interact with WindowServer at all).
It'll also be useful for notification windows and the taskbar, which
currently both rely on the GUI::Desktop callback but will now be able to
listen and react to the event themselves.
WindowServer now collects applet windows into an "applet area" which is
really just a window that a WM (window management) client can position
via IPC.
This is rather hackish, and I think we should come up with a better
architecture eventually, but this brings back the missing applets since
the global menu where they used to live is gone.
Window icons in Taskbar were previously received in WM events with
shbuf ID's. Now that Gfx::ShareableBitmap is backed by anonymous files,
we can easily switch to using those.
