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. :^)

Libraries/LibCore/CProcessStatisticsReader.h

@@ -24,6 +24,7 @@ struct CProcessStatistics {
     unsigned ticks;
     String priority;
     unsigned syscall_count;
+    int icon_id;
 
     // synthetic
     String username;