New comparison dialog

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

The much maligned New dialog is now gone. In current Git, 'File->New
comparison' now opens a new tab with the relevant UI for starting a
new comparison; this tab is also shown by default if you start Meld
without any arguments.

I'm hoping that this new tab will improve the initial experience, and
will also reduce user error when selecting files and folders for
comparison.

There are several differences to the old dialog, most notably that the
new tab uses standard GTK+ file selectors, and so doesn't support
selecting files from history. However, with the new support for
recently-used comparisons, I feel that this isn't that big a problem.

While it's very unlikely that the old dialog will make a comeback, I'm
keen to hear about any issues people have with the new tab, as well as
possible improvements.

cheers,
Kai