Simplify code introduced in 7f7ea810.
Remove unused method in panel API.
The smart way didn't work, keep it simple.

When jQuery wasn't loaded yet, the <script> tag was inserted properly,
but jQuery wasn't loaded before jquery.cookie.js and toolbar.js. As a
consequence these scripts crashed.

Refs #440.
Restore XML validity.
Avoid some implicit global lookups.

They made it impossible to preserve panel data after the end of a
request.
Load the content of panels dynamically.

This should drastically reduce the overhead of the browser toolbar when
a page has a complex template structure or many SQL queries.

This change is backwards-incompatible for third-party panels because it
changes the signature of __init__.

The JavaScript could probably be improved; I'm outside my comfort zone.
Improve regression test added in 8525bfc7.

Refs #422.
Invalidate cached list of panels during tests.
Move a test specific to the profiling panel in its module.
Move a test specific to the template panel in its module.
Normalize the way we write panel tests.
Move two tests specific to the request vars panel in their module.
Fix PEP 8 violations.