Added Save and Load management to ImagesList and emitted:

signalXMLSaveItem, signalXMLLoadImageElement,
signalXMLCustomElements, signalXMLCustomElements
to save and load custom attributes and elements.
File format is XML:
<?xml version="1.0" encoding="UTF-8"?>
<Images>
    <Image url="file:///path/to/image" custom_attr1="value1">
        <CustomImageElem CustomImageElemAttr="attribute value"/>
    </Image>
    <CustomElem customElemAttr="" />
<Images/>

I suggest plugins to add a plugin prefix to allow loading imagelist
also to those plugins that don't have custom elements/attributes.

CCMAIL=digikam-devel@kde.org,kde-imaging@kde.org