ModalWindow panel support

[Martijn Dashorst <ma...@gmail.com>]

My co-workers have been working on a solution to fix ModalWindow's
Panel support. The problem with Panels inside a ModalWindow is that
they break CSS rules, and have crappy form submit support. Therefore
until now, it has been advised to use Pages instead inside
ModalWindow, as iframes break the markup hierarchy, fixing the CSS
problems and the form submit behavior.

But using a Page inside a ModalWindow is problematic in its own
regard: it takes an extra request to show the ModalWindow (extra
latency and processing time), you have to cater extra CSS for these
pages, and it requires another pagemap.

The solution my coworkers came up is to generate a <div> at the end of
the page, ensuring the <div> is detached from the CSS flow, and
generating all markup for the ModalWindows inside that div. This fixes
the CSS rules (as long as you don't target *all* divs in your CSS, but
we can even generate a <div style="overwrite everything"> to overcome
such malice), nested forms are no problem anymore, etc.

This works great in our app, and we're phasing out all our Page based
MWs. Our solution is not generic enough for direct inclusion into
Wicket, but I figured we could strive to implement this in 1.5, making
MW support even better and more consistent.

I'll ask our devs to create a quickstart or even provide a patch, if
there are no objections to this generic description.

Martijn