Search option for opening branches

### Problem

When opening a branch from the context menu, the list is sometimes very long and the full branch names can also be long so it is hard to find the required one. Often part of the name is known but not the full name.

### Proposal

Have a "Search" option in the Branch context menu opening a dialog allowing a partial name to be entered and then the local and remote (if necessary) branches to be searched for matching entries. This could in principle replace the present "Local" and "Remote" and "Create" options. Such a "BranchChooser" dialog could be used in future for other functions.

### Prior Art (Optional)

On Github, the "Code" tab has a branch dropdown that includes a search entry with placeholder "Find or Create a Branch".