Add support for interactive rebase; and option - reword

gitg now has a command line argument --rebase-editor.
Starting gitg thus will invoke `git rebase -i M~N`
on the currently checkedout branch for the current repository,
where N is derived from the row that was selected before
interactive rebase was initiated. M currently is fixed to be
HEAD.
Parsing git-rebase-todo
Split git-rebase-todo file into lines and spit
Slice the git-rebase-todo array into 3 cols and n rows
Added definition for rebase listbox