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/WindowMaker.h

@@ -426,6 +426,7 @@ extern struct WPreferences {
     char cycle_ignore_minimized;        /* Ignore minimized windows when cycling */
     char strict_windoze_cycle;          /* don't close switch panel when shift is released */
     char panel_only_open;               /* Only open the switch panel; don't switch */
+    char apercu_size;                   /* Size of apercu preview as a multiple of icon size */
 
     /* All delays here are in ms. 0 means instant auto-action. */
     int clip_auto_raise_delay;         /* Delay after which the clip will be raised when entered */