magit-branch-manager: remove kludge

`magit-mode-setup' now takes care of this.
magit-branch-manager-mode: doc-string cosmetics
magit-wash-branch-line: use magit-bind-match-strings
magit-bind-match-strings: add STRING argument
magit-insert: support inserting already propertized string
refactor the branch manager

This certainly has to be further improved, including but not limited
to how the information is presented.  But that can wait until later.
show list of tags in the branch manager
magit-branch-edit-description: new command
magit-insert-branch-description: new function
Merge branch 'n/branch-manager-1' into next