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.

src/screen.h

@@ -27,6 +27,9 @@
 
 #include <WINGs/WUtil.h>
 
+#define PRINT_SCREEN 1
+#define PRINT_WINDOW 2
+#define PRINT_PARTIAL 3
 
 typedef struct {
     WMRect *screens;
@@ -271,6 +274,11 @@ typedef struct _WScreen {
     WMHandlerID *workspace_name_timer;
     struct WorkspaceNameData *workspace_name_data;
 
+    /* mini screenshot data */
+    Window mini_screenshot;
+    time_t mini_screenshot_timeout;
+    WMHandlerID *mini_screenshot_timer;
+
     /* for raise-delay */
     WMHandlerID *autoRaiseTimer;
     Window autoRaiseWindow;	       /* window that is scheduled to be
@@ -314,7 +322,7 @@ void wScreenRestoreState(WScreen *scr);
 
 int wScreenBringInside(WScreen *scr, int *x, int *y, int width, int height);
 int wScreenKeepInside(WScreen *scr, int *x, int *y, int width, int height);
-
+void ScreenCapture(WScreen *scr, int mode);
 
 /* in startup.c */
 WScreen *wScreenWithNumber(int i);