Add "applyDefaultIcon" setting

[ahigerd]

Some applications don't provide their own icons. By default, Openbox sets a generic icon for these applications. Other environments, such as GNOME, instead supply an icon from /usr/share/pixmaps or the icon theme. Openbox's behavior is appropriate when it is running on its own, but when it is being used as the window manager in a broader desktop environment, the presence of the default icon prevents the environment from noticing that it needs to provide an icon.

This change adds an <applyDefaultIcon> configuration option that defaults to yes to preserve the standard Openbox behavior. By setting this option to no, Openbox will cede responsibility for setting default icons to the desktop environment.

Resolves issue #21