Fixes issue #620 selection dialogs for branches and bookmarks

The goal was to help users enter branch names and bookmark names when in dialogs such as the Push dialog.

I'm not aware of other places where such dialogs would be useful, but if there happened to be such places in the plugin, it would be a trivial task to use the dialogs.

One option could have been, given that the dialogs list all existing open branches and all existing bookmarks, to disable the branch and bookmark text fields. I chose not to do that because the goal is to help users, not coerce them.

NOTE 1: I don't know how Heptapod acts on an open branch when the branch gets merged, i.e. whether the branch is closed before the merge or whether we'll find ourselves with the merge-then-close-or-close-then-merge problem again. The documentation has screen captures of a "Delete source branch" but I guess it's available for reviewers of merge requests, not for creators.

**NOTE 2: I have not used topics for this merge request, due to topics requiring the Evolve extension and the bad experience that I've had with Evolve polluting the command server in JavaHg with extraneous warning and error messages. This MR is based on a standard branch as was the case with Bitbucket.