Add option to allow starting DockApps with a single click

This patch is from the contrib directory. It adds an option to allow starting
DockApps with a single click. It's a handy option that adds only a few lines to
the code.

It is a good feature patch example because it includes modifications
to the WPrefs application so that the feature can be easily enabled or
disabled. The one-click ability allows wmaker to integrate more seemlessly with
programs like ROX-Filer which can be configured to use single or double click
actions.

The README in the contrib/ directory says,

author:
Sebastien Bauer <seb_bauer@bigfoot.com>
John Morrissey <jwm@horde.net>

updated for Window Maker 0.65.0 by:
Daniel Richard G. <skunk@graphics.lcs.mit.edu>

updated for Window Maker 0.80.2 by:
Martial Daumas <martial@nasgaia.org>

update for 0.80.2+ by:
steve lion <steve.lion@verizon.net> and vlaad

src/defaults.c

@@ -474,6 +474,8 @@ WDefaultEntry optionList[] = {
 	 &wPreferences.attract, getBool, NULL},
 	{"DisableBlinking", "NO", NULL,
 	 &wPreferences.dont_blink, getBool, NULL},
+	{"SingleClickLaunch",	"NO",	NULL,
+	 &wPreferences.single_click, getBool, NULL},
 	/* style options */
 	{"MenuStyle", "normal", seMenuStyles,
 	 &wPreferences.menu_style, getEnum, setMenuStyle},