Codechange: use better location for the "invalid" action sentinel value

It used to be a random sentinel for end-of-(widget-)list that was used to tell
that no action has taken place yet. Since the last action is practically the
widget that was pressed, add the sentinel to that enumeration.

src/widget_type.h

@@ -17,8 +17,6 @@
 #include "gfx_type.h"
 #include "window_type.h"
 
-static const int WIDGET_LIST_END = -1; ///< indicate the end of widgets' list for vararg functions
-
 /** Bits of the #WWT_MATRIX widget data. */
 enum MatrixWidgetValues {
 	/* Number of column bits of the WWT_MATRIX widget data. */