Children for strings

I added 3 lines/URLs to my shelf a few weeks ago to paste them quickly when needed, but it suddenly dawned on me that there were times when I only wanted to paste one of the lines.

It struck me as strange that nobody's ever brought up the idea of children for strings - each line, much like text files have the lines as their children.

This pull adds the ability to → into multi-line strings and get each line as a new object.

Things about this pull:

* I've used a different method (from [here](http://www.karelia.com/cocoa_legacy/Foundation_Categories/NSString/Split_Into_LInes.m)) for splitting strings as opposed to a built in Apple method since it seems more reliable
* I've added a nice new block convenience method for creating new arrays based on the contents of old arrays :)
* I've now given string objects identifiers. They never had them before. Initially I thought this might break things somewhere, but I've realised it might actually fix #1504 - since every string object is now unique
* Previously, even if `objectHasChildren` returned `NO` for a given object, its children would still be shown when →. I've changed the `browse:` method so that this BOOL is honoured before attempting to load children. It means that you no longer need to do `objectHasChildren` logic in the `loadChildrenForObject` method