Kernel+ProcessManager: Let processes have an icon and show it in the table.

Processes can now have an icon assigned, which is essentially a 16x16 RGBA32
bitmap exposed as a shared buffer ID.

You set the icon ID by calling set_process_icon(int) and the icon ID will be
exposed through /proc/all.

To make this work, I added a mechanism for making shared buffers globally
accessible. For safety reasons, each app seals the icon buffer before making
it global.

Right now the first call to GWindow::set_icon() is what determines the
process icon. We'll probably change this in the future. :^)

Kernel/FileSystem/ProcFS.cpp

@@ -548,6 +548,7 @@ ByteBuffer procfs$all(InodeIdentifier)
         process_object.set("ticks", process.main_thread().ticks());
         process_object.set("priority", to_string(process.priority()));
         process_object.set("syscall_count", process.syscall_count());
+        process_object.set("icon_id", process.icon_id());
         array.append(process_object);
     };
     build_process(*Scheduler::colonel());