Match placeholders in Objective-C strings

We do this by explicitly matching ‘%@’ (with potential flags and similar) in the Objective-C grammar, and then include the ‘string_placeholder’ rule from the C grammar.

Also switch the rule for matching escaped characters (in Objective-C strings) with an include (for re-using the C grammar’s rule).
Match ‘@package’ keyword
Remove array iteration snippet

This snippet is obsolete as one should iterate arrays with the “for … in” syntax (for which we already have a snippet).
Update int → NSInteger for NSAlert result
Use an Objective-C literal instead of NSDictionary
Update property snippet for ARC

Prior to ARC, the default storage for retainable pointer types was ‘assign’ meaning you pretty much had to set a storage type.

With ARC, the default storage type (for retainable pointer types) is now ‘strong’ meaning you probably want to leave out the storage type. Therefor I made the pop-up default to ‘readonly’ and deleting it will also delete the comma. You can still find ‘retain’ and friends in the pop-up (which now also include ‘weak’).