Formula engine: change undocumented string substitution syntax

It used to use {}, which gets caught by the preprocessor.
Now it uses [] instead.

data/gui/default/widget/toggle_button_orb.cfg

@@ -9,7 +9,7 @@
 		y = 0
 		w = {SIZE}
 		h = {SIZE}
-		name = "('buttons/misc/orb{STATE}.png" + <<~RC(magenta>{icon})')>>
+		name = "('buttons/misc/orb{STATE}.png~RC(magenta>[icon])')"
 	[/image]
 #enddef
 

src/formula.cpp

@@ -558,8 +558,8 @@ public:
 		subs_()
 	{
 		std::string::iterator i;
-		while((i = std::find(str.begin(), str.end(), '{')) != str.end()) {
-			std::string::iterator j = std::find(i, str.end(), '}');
+		while((i = std::find(str.begin(), str.end(), '[')) != str.end()) {
+			std::string::iterator j = std::find(i, str.end(), ']');
 			if(j == str.end()) {
 				break;
 			}