Add a screenshot capture feature

This patch adds a feature to take screenshots directly from Window Maker.
Having the feature embeded direclty inside Window Maker allows us to take
advantage of how Window Maker is managing and handling Windows.
Three new actions can be bound to a key shortcut from WPrefs.
The screenshot files are saved in ~/GNUstep/Library/WindowMaker/Screenshots/
dir, with a "screenshot_%Y-%m-%d_at_%H:%M:%S" format followed by the
extension. Preferably as a PNG or JPG file if available.
Meaning, to work Window Maker via WRaster needs to support
at least one of those format.

"Capture the entire screen" is quite standard, it takes a screenshot
of the whole screen area (even in multiheads env).
"Capture a portion of the screen" requires the user to draw a rectangle which
will be captured.
Those two first are quite straightforward, just taking a live picture of
the screen.
The last one is "Capture a window" which works in best effort mode,
it catures the focused window. As Window Maker by default is not using
any compositor (like for example Xcompmgr) it can only dump the content
displayed on the screen.
If a window is minimized or out of the screen, there is high chance the
image will be split or some area greyed in case other windows overlapped it.

WPrefs.app/KeyboardShortcuts.c

@@ -158,7 +158,10 @@ static struct keyOption {
 	{ "RunKey",            N_("Run application") },
 	{ "ExitKey",            N_("Exit Window Maker") },
 	{ "DockRaiseLowerKey", N_("Raise/Lower Dock") },
-	{ "ClipRaiseLowerKey", N_("Raise/Lower Clip") }
+	{ "ClipRaiseLowerKey", N_("Raise/Lower Clip") },
+	{ "ScreenCaptureKey", N_("Capture the entire screen") },
+	{ "WindowCaptureKey", N_("Capture a window") },
+	{ "PartialCaptureKey", N_("Capture a portion of the screen") }
 #ifdef XKB_MODELOCK
 	,{ "ToggleKbdModeKey", N_("Toggle keyboard language") }
 #endif				/* XKB_MODELOCK */