Theme Forms API (allow themes to modify FAPI forms)

To get this group kick started, I thought I would post my own recent idea for a new module. It’s still in the proposal stage and has no code, but early feedback will definitely help its development…

I started thinking about allowing active themes to modify forms while working on the custom theme settings api (also at #57676).

To avoid namespace collisions with modules, I was thinking THEME_theme_form_alter(), ENGINE_theme_form_alter(), and ENGINE_engine_form_alter(). Where “THEME” is replaced with the theme name (like “zen”) and ENGINE is replaced with the theme engine name (like “phptemplate”).