ImageManager needs config option to show/hide "advanced" options

[gogo]

(Tracking ticket, commit to follow).

ImageManager offers a number of "advanced" options when inserting images, namely margins, padding, padding/background colour, borders and border colour, alignment.

These options are those which can not be determined from the image as given by the URL itself - as opposed to width and height (remembering that ImageManager automatically scales the image when the dimensions are changed).

It is useful to be able to disable these advanced options in the ImageManager, both within Xinha for design considerations, and also when one hijacks the ImageManager plugin to serve as a generic Image Picker returning a url to a text field.

$IMConfig['show_full_options'] = false;

Defaults to true of course.

[gogo]

changeset:830