(svn r6542) -Feature: [depot window] added a "sell all vehicles in depot" button

It's right below the sell button (sell whole chain button for trains)
   It's still missing a sprite. That one will be added as soon as anybody draws something we can use
   To make room for this button, all depots except train depots now opens with an additional row and can't be resized shorter than that

openttd.h

@@ -439,6 +439,7 @@ enum {
 	WC_GENERATE_LANDSCAPE       = 0x50,
 	WC_GENERATE_PROGRESS_WINDOW = 0x51,
 	WC_OK_CANCEL_QUERY          = 0x52,
+	WC_DEPOT_SELL_ALL           = 0x53,
 };