wmaker: allow the screenshot filename template to be set

This patch is adding a new option ScreenshotFilenameTemplate for users
to change the default strftime format "screenshot_%Y-%m-%d_at_%H:%M:%S".
For example, Teams is not allowing the ':' char in the filename of file
to be shared so I always have to rename the file.

src/WindowMaker.h

@@ -505,6 +505,7 @@ extern struct WPreferences {
 	Cursor cursor[WCUR_LAST];
 
     int switch_panel_icon_size;               /* icon size in switch panel */
+    char *screenshot_filename_template;       /* strftime format for screenshot filenames */
 
 } wPreferences;