(svn r10622) -Feature: Introduction of the CBID_INDUSTRY_AVAILABLE's handling, in fund industry window.

In order to stay as much consistent with the specs, the fund window will perform (if required) the callback every game-day.  TTDPatch performs the same call every second, but after discussions with Csaboka, we agreed that it was not required to be that intensive.  So a game-day is plenty enough.

src/newgrf_callbacks.h

@@ -75,7 +75,7 @@ enum CallbackID {
 	CBID_HOUSE_DESTRUCTION               = 0x21,
 
 	/** Called to determine if the given industry type is available */
-	CBID_INDUSTRY_AVAILABLE              = 0x22, // not implemented
+	CBID_INDUSTRY_AVAILABLE              = 0x22,
 
 	/** This callback is called from vehicle purchase lists. It returns a value to be
 	 * used as a custom string ID in the 0xD000 range. */