Replace instances of std::map<std::string, std::string> with StringMap

Also, clean up surrounding code style
Replace by-value parameter passing with const refs when possible
Fix post-increment of iterators

src/guiEngine.h

@@ -73,7 +73,7 @@ class TextDestGuiEngine : public TextDest
 	 * receive fields transmitted by guiFormSpecMenu
 	 * @param fields map containing formspec field elements currently active
 	 */
-	void gotText(std::map<std::string, std::string> fields);
+	void gotText(const StringMap &fields);
 
 	/**
 	 * receive text/events transmitted by guiFormSpecMenu