util: add GDK helper library

Added function to fetch supported EWMH properties.

Based on example code from
http://opensourcejavaphp.net/csharp/gtksharp/windowmanager.c.html
main: only set override redirect if EWMH is unsupported
main: use correct atom name (_NET_WM_STRUT)
main: handle both strut and strut_partial properties
Use correct indexes on strut partial array
Use correct name for net_wm_strut_partial_atom

Sigh.
Merge branch 'fix/docking' into develop

Fixes #47