Fix exception when modifying immutable recent dict
Fix exception when modifying immutable recent dict

SKBookmarkController assumes that all items of recentDocuments are instances of
NSMutableDictionary, however -[NSUserDefaults persistentDomainForName:] no
longer returns mutable leafs on OS X 10.10.

nanoant/Skim,nanoant/Skim,nanoant/Skim,nanoant/Skim,nanoant/Skim