Update to use ARC and modernize

There should be no functional changes. I encountered an issue with things being over-released when using nibs after initially updating the code to ARC, but [this](https://github.com/rdwampler/dialog-1.x/blob/7219be7dce7cd06b9ad2d8efd854ad627ff5f262/Dialog.mm#L168) seemed to have resolved it. 

I am planning on submitting a similar PR for the `tm_dialog2` shortly, if you do not find any issues with this one, as the changes/fixes are essentially the same.