This is a convenience helper to instantiate a GIcon like so:
auto icon = GIcon::default_icon("filetype-image");
This will give you the "filetype-image" icon in both 16x16 and 32x32 sizes.
It's basically a userspace port of the kernel's Lock class.
Added gettid() and donate() syscalls to support the timeslice donation
feature we already enjoyed in the kernel.
These use nearest neighbor and are computed synchronously on directory load
so it's neither fast nor very beautiful. These issues both need work on
other parts of the system to fix.
