Move IPE html wrapper code in a theming function

In short: this patch refactors some more hardcoded markup from the panels_renderer_ipe.class.php into a theming function which can be more easily overwritten.

We needed to alter the markup of the Panels-IPE pane wrapper for better support of float based gridsystems. Our Grid broke when being logged in and using the IPE dragging around elements. (I'm aware of creating my own renderer plugin but still the markup shouldn't be there)