Settings menu improvements regarding default values (#13489)

The reset button now removes the setting from minetest.conf instead of setting it to its default value.
The reset button is now shown whenever a value is present in minetest.conf
Float settings now get a .0 suffix if they have no decimal places.

src/script/lua_api/l_settings.h

@@ -59,6 +59,9 @@ private:
 	// get_names(self) -> {key1, ...}
 	static int l_get_names(lua_State *L);
 
+	// has(self, key) -> boolean
+	static int l_has(lua_State *L);
+
 	// write(self) -> success
 	static int l_write(lua_State *L);