Add GUI to allow users to select from multiple FileFormats.

When multiple FileFormats can read/write a file, let the user
choose which to use. Also, force the built-in formats when saving a
file (as opposed to exporting it).

Also split open into Open (our cml/cjson) and Import (all formats) and
clean up various QFileDialog usage issues (initial dirs, user cancels).

Change-Id: Id33aa3a981ab42661578cfddb0db59699dbba8da
Read/write molecules in a background thread.

Change-Id: I28b7e4789f7afb48280c09e785ff3bae52dea83c
Merge topic 'file-io-selection' into master

41d09e72 Read/write molecules in a background thread.
8c6e35dd Add GUI to allow users to select from multiple FileFormats.