apercu: Make preview size a configuration option

Since the resolution of the Retina display tends to make everything small,
the default apercu preview size (twice the icon size) couldn't be used to
distinguish the window contents without tiring too much my eyes.

Therefore, let's make the apercu size a configurable option. You can set
it through the ApercuSize variable with

$ wdwrite WindowMaker ApercuSize 4

in multiples of the icon size (in this case the apercu size will be four
times the icon size).

The default size remains 2 (twice the icon size).

src/defaults.c

@@ -479,6 +479,8 @@ WDefaultEntry optionList[] = {
 	    &wPreferences.strict_windoze_cycle, getBool, NULL, NULL, NULL},
 	{"SwitchPanelOnlyOpen",	"NO",	NULL,
 	    &wPreferences.panel_only_open, getBool, NULL, NULL, NULL},
+	{"ApercuSize", "2", NULL,
+	    &wPreferences.apercu_size, getInt, NULL, NULL, NULL},
 
 	/* style options */