Added ability to provide function to find to find by other than id

[tjcouch-sil]

Added ability to provide a function to dockLayout.find instead of just an id. The function's first parameter is the box, panel, or tab being considered. The function returns true if the box/panel/tab matches and should be returned by the find function.

I called the parameter 'item' because I didn't know what is normal to call that kind of thing. Whatever name makes sense sounds good to me.

I updated find's signature but not the JSDocs.

I added this because I needed a way to search for a tab by function. This implementation is not necessarily the best way to do this (you're free to change it as you'd like), but this was just an easy way I thought of that would also keep backward compatibility.

Thanks for the hard work on rc-dock!!