Refresh allowed contents even if edit cache of panels display is set

Problem/Motivation

Currently the allowed_contents for a panels_display are cached in the edit cache. This makes it fast, however it's not so thrilling if you work with multiple tabs e.g. you change a page manager page and you realize that you should put stuff in a mini panel to make it reusable.
So you open a new tab, create the mini panel and then you'd like to add that mini panel to the page manager page.
Unfortunately the newly created mini panel won't show up in the content-dialog until you saved or / cancelled the page manager page.

Proposed resolution

Re-new the allowed content list even if the panels display is fetched from the cache.
The patch also contains a typo fix in the pane node edit cache.

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

None.