Support _NET_WM_PID

[Earnestly]

When using tools like wmctrl -lp the PID for sxiv windows are displayed as 0 indicating that "the application owning the window does not support it."

For example:

$ wmctrl -lp
0x01e00001  0 0         N/A sxiv
0x0220000d  0 999255 teapot wmctrl -lp

This makes it more difficult to assign or change the window and its properties through scripts as multiple instances can't be differentiated so simply. I assume this is due to the lack of _NET_WM_PID.