[ui] add a status bar button to initiate checksum computation

* Just click the "hash" sign to get a hash
* Also clean up status bar related functions

src/rufus.h

@@ -152,6 +152,15 @@ typedef struct {
 	Callback_t callback;
 } notification_info;	// To provide a "More info..." on notifications
 
+/* Status Bar sections */
+#define SB_SECTION_LEFT     0
+#define SB_SECTION_MIDDLE   1
+#define SB_SECTION_RIGHT    2
+
+/* Distance, from the right border, of the Status Bar separators */
+#define SB_EDGE_1           77.0f
+#define SB_EDGE_2           58.0f
+
 /* Timers used throughout the program */
 enum timer_type {
 	TID_MESSAGE_INFO = 0x1000,