Option to hide stale branches

[Uncommon]

• [ ] Add a control (in the sidebar or toolbar somewhere) to toggle hiding branches exceeding a certain age, going by the commit date on the head commit
• [ ] Dim old branches when they are shown
• [ ] Don't filter out the current branch or its tracking branch
• [ ] Indicate the number of hidden branches
• [ ] Also filter the toolbar popup
• [ ] Also filter branches shown in history
• [ ] Preference setting for how old is old