Fix Save Preview As... for textures, now save as any type we can encode to!

Removes the previous method and condenses the menu entries back into one.
 - Translations have been updated except for German, German will need to update the translation for "Save Preview As..." since the name hasn't changed.
Adds FFSAVE_IMAGE to the filepicker enums
Crossplatformity:
 - The Windows version works nicely
 - The Linux version may not work or even compile but it should..
 - The Mac version hasn't been added, instead it will default to accept any(all filter), should the provided file extension be invalid a notification will display.

indra/newview/statemachine/aifilepicker.cpp

@@ -329,6 +329,9 @@ void AIFilePicker::open(std::string const& filename, ESaveFilter filter, std::st
 		case FFSAVE_PHYSICS:
 			mFilter = "physics";
 			break;
+		case FFSAVE_IMAGE:
+			mFilter = "image";
+			break;
 	}
 }