feat: Added option to prune outdated refs during fetch

[kosmakoff]

This change adds options to Fetch dialog to prune outdated remote references, i.e. those remotes that do not exist anymore.

[love-linger]

#995

You can enable global fetch.prune in Preference window.

For these reasons, this PR will not be merged

• This PR is the same as the implementation of SourceGit before 8.35. The reason why it is changed to the current state is due to some feedback and consideration.
• In this PR, when Prune outdated remote refs is unchecked, if fetch.prune is enabled in global/local(repository), the actual behavior does not match the design

[kosmakoff]

Understandable, I will close the PR now.