iOS: enable nifty-gui support (#269)

Without the ProGuard exceptions at least these elements are not usable:
ButtonBuilder, LabelBuilder, TextFieldBuilder

jme3-ios/ios-data/templates/ios.properties

@@ -44,6 +44,10 @@ ios.proguard.options=-keep public class com.jme3.system.ios.*{public *;} \
 -keep public class * implements com.jme3.asset.AssetLoader{public *;} \
 -keep public class * implements com.jme3.asset.AssetLocator{public *;} \
 -keep public class * implements de.lessvoid.nifty.screen.ScreenController{public *;} \
+-keep public class de.lessvoid.nifty.loaderv2.types.** { public *;} \
+-keep public class de.lessvoid.nifty.controls.** { public *; } \
+-keep public class de.lessvoid.nifty.input.** { public *; } \
+-keep public class de.lessvoid.nifty.effects.impl.** { public *;}
 -keepclassmembers class com.jme3.audio.plugins.NativeVorbisFile{public *;} \
 -keep public class * implements javax.xml.parsers.SAXParserFactory{public *;} \
 -keep public class com.sun.org.apache.xerces.internal.jaxp.SAXParserFactoryImpl \