Change mainmenu texture handling + small misc changes

Texture names must now be escaped in formspec elements image[],
background[], image_button[], image_button_exit[].

Instead of special-case handling of texture loading (and unloading
which was missing) in guiFormSpecMenu.cpp, use the newly created
ISimpleTextureSource interface which is a minimal subset of
ITextureSource. There is an implementation of this interface
used by GUIEngine (MenuTextureSource).

Fix an off-by-one bug in unescape_string; it caused requests for a
texture called "\0".

src/util/string.h

@@ -307,7 +307,7 @@ inline std::string unescape_string(std::string &s)
 {
 	std::string res;
 	
-	for (size_t i = 0; i <= s.length(); i++) {
+	for (size_t i = 0; i < s.length(); i++) {
 		if (s[i] == '\\')
 			i++;
 		res += s[i];