fix (js) : Avatar cropper positionning bug

There is a positionning bug on the avatar image crop selection, at least in Chrome (but not in Firefox) : when the page is scrolled, the selection appears under the image, with a offset that increases proportionnally to the scrolling.

Tested on Chromium Version 32.0.1700.107 Ubuntu 12.04, but also reported on (unknown) Chrome versions under Mac OSX and Windows.

This involves a jquery plugin called imgareaselect ; according to this plugin's documentation and forums, this can be resolved by setting the "parent" param to a closer container, that has to be positionned "relative" or "absolute" (no float, no fix, no static).
Also, it requires to set the z-index, and it appears  that setting "movable" to true also helps make it work properly on Chrome.

- [ ] Change commit message to [standard format][1]
- [ ] resubmit to 1.9 branch

[1]: http://learn.elgg.org/en/latest/contribute/code.html#commit-message-format