Patch to provide Hook for Default Modes + Export

Here's some initial work that adds Exporting Modes to Code, and implements hook_mode_defaults().

Adds a second table to the modes admin page for "Default" modes, with just an Import link. That link mimics views behavior, it loads up the default mode code and puts it as default values in the Mode Add form.

This is very initial code, eventually would want to fully integrate the normal/default/overridden/revert paradigm, but for now, its a nice way to get permissions into code.