Reflow commit messages in diff view

This aims to reformat commit messages in the diff view to look better
when presented in a GUI such as gitg. Hard-wrapping is undone so that
lines are soft-wrapped naturally in the available space, while trying
to preserve intentional formatting such as lists, indentation and
paragraphs.
Added initial action interfaces in GitgExt
Added ref delete/rename action stubs
Implement history navigation as ListBox
Rename GitgExt.RefAction to GitgExt.Action
Add action interface implementation
Implement GitgExt.Action interface
Improve selection and implemented editing
Remove obsolete style
Generalize info bar to user query
Added GitgExtUserQuery
Implemented animations for refs list
Add clearing repository refs cache
Cleanly separate ref actions using interfaces
Fix install targets
Set infobar action area to vertical orientation
Clear ref cache in action interface
Implemented delete action for tags and branches
Fix event cast
Add reflog messages