Performance: panels_page_admin_menu_items

panels_page_menu doesn't play nice with the menu system: It adds 13 menu items per panel page + 3 per secondary display to the cacheable part of the menu system. 10 panel pages on the system may well add up to 150 additional items that are loaded on every page.
Sure, mostly the menu system will handle a lot of menu items pretty well. But the used memory is noticeably increased, and pages like admin/build/menu perform incredibly slow with many items.

A performance test on my (really slow) local server revealed for admin/build/menu:

0 additional menu items:

Page execution time: 1415.06 ms, 1243.72 ms, 1334.74 ms.

Memory used at devel_shutdown(): 11.23 MB

500 additional menu items:

Page execution time was 5477.54 ms, 5121.97 ms, 5093.85 ms.

Memory used at devel_shutdown(): 11.87 MB

2.000 additional menu items:

Page execution time: 58692.01 ms, 57299.53 ms, 57352.56 ms.

Memory used at devel_shutdown(): 13.78 MB

We don't need to be that resource hungry. Especially Panels' admin functionality shouldn't have such an impact on the system on other pages. We can easily solve this by moving the items to the non-cached part, using the method described in #266907: Unnecessary and wrong query on several admin pages. I'll provide a patch, as soon that other issue has been RTBC'ed.