This commit turns the icon handling into using a macro so we don't need to do
weird conversions from unsigned long (needed by X11) to uint32_t (needed by
Wayland).
Also simplified the error handling path on Wayland.
Many X11 window managers will present an application on their
taskbar as a combination of the application name and an icon
imagery pulled from the X-Property _NET_WM_ICON. Applications
built under frameworks such as Qt or GTK have this property
populated by the framework. This commit adds the Atom _NET_WM_ICON
and populates it with a 64x64 icon of Looking Glass.