Change donate link to our own redirection

As discussed in #410, the previous SF redirect is better replaced by
a redirect on www.geany.org which we can control.
This way we can also quickly change the target URL if it changes at
Paypal.

src/geany.h

@@ -45,7 +45,7 @@ G_BEGIN_DECLS
 #define GEANY_HOMEPAGE					"http://www.geany.org/"
 #define GEANY_WIKI						"http://wiki.geany.org/"
 #define GEANY_BUG_REPORT				"http://www.geany.org/Support/Bugs"
-#define GEANY_DONATE					"https://sourceforge.net/donate/index.php?group_id=153444"
+#define GEANY_DONATE					"http://www.geany.org/service/donate/"
 #define GEANY_STRING_UNTITLED			_("untitled")
 #define GEANY_DEFAULT_DIALOG_HEIGHT		350
 #define GEANY_WINDOW_DEFAULT_WIDTH		900