Break RepositoryView repos refresh into 3 parts to not block UI

The Central.refreshPlugins() call could lock up the UI thread if longer
running refreshing would happen.

Also, stopped redundant doRefresh() calls from happening due to
multiple NewBuilder.build() isCnfChanged() checks causing the
refreshing to happen multiple times unnecessarily.

Signed-off-by: Carter Smithhart <carter.smithhart@gmail.com>
Merge pull request #979 from derheld42/repositoryview_no_block_ui

Break RepositoryView repos refresh into 3 parts to not block UI