Removed deprecated `startfile` interpreter option.

Use `startwith` option instead.
Added explicit sorting of files in the filetree.

Some systems (Linux and some instances of OSX) seem to return files in
arbitrary order.
Fixed find dialog to stay on top after search directory selection on OSX.

OSX moves the find dialog to the background after using the directory
dropdown in the open file dialog. This workaround keeps find dialog on top.