Renderable elements enables you to register any forms and manage the display through an UI. It will also make (additional) elements available of existing entities on the manage forms/display screens or you can for example register the contact form and rearrange the fields through Field UI.
Renderable elements is a very experimental module touching the bounderies of form alter, some contrib modules may clash immensively, so use with care.
Be care on special elements when you're choosing to render: #1889036: Mark/preserve elements may lead error with other modules
Installation
Once you've enabled the module, you'll see a 'Manage form display' link on top of any form. Clicking on it will register it which you'll find on the overview at admin/structure/rel.