Adds a mechanism through which windowing clients can re-request an
UpdateSystemTheme message. This is currently used in SystemMenu's
ShutdownDialog to refresh it's theme when the dialog is instantiated.
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.
This patch replaces the use of shbufs for GUI::Window backing stores
with the new anonymous files mechanism.
Backing bitmaps are now built on memory allocated with anon_create().
They are passed across the IPC socket as IPC::File. This means that
WindowServer now pledges "recvfd" and graphical clients need to pledge
"sendfd" to work.
To support the cached bitmap swapping optimization on the WindowServer
side, each backing store is assigned an incrementing serial number on
the client side. (This allows us to re-use an already mapped file.)
Instead of storing the back/front buffers of a GUI::Window as just
Gfx::Bitmap, wrap them in a WindowBackingStore class.
This will allow us to add more information alongside the bitmaps while
keeping the back/front swapping logic simple.
