Add branch dropdown

This new feature has the purpose to have a list of all the branches in a place and allow to easily checkout a desired branch. This component is based on the "remote" component. It used the same UI and a big part of the code. 
For the moment, no tests are implemented. 
In the future, it would be great to add a search box to this component with an autocompletion, so it will be easier to   search a branch if you have a lot of branch. 
Why are we changing this file?  I don't think you meant to change this file as newly added feature does not require any changes here.

And if you want to make changes here but to keep it out side of git repo you can do this via [config file](https://github.com/FredrikNoren/ungit#configuring).