Update for better stability of avatar cropper

Updated patch has evolved after your comments :
- changed 'parent' param to the form itself, rather than body (default), so it depends less on the theme CSS, and relies on a more stable element that a 'div.clearfix'. Also attaching an editing tool to the edition form itself seems more logical than to the page body or an inner element.
- kept 'movable:true', it's default value anyway, but is useful to solve Chrome bug when it happens (depending on theme CSS).
- kept 'zIndex:20' : 20 is arbitrary, could be less.. also useful for Chrome.
- added 'handles', which visually helps to know one can resize the selection box.
- added an optional param + notice for keyboard control. Can be useful in some cases, but as it maps the Arrows + Ctrl/Shift/Alt keys i think it's not a good idea to activate it by default.
- also forced form position directly in the JS file : this make it certain that the correct positioning is used for the cropper to display properly. Also it avoids a CSS patch, and ensures it doesn't depend neither on core nor theme CSS (more stability).