Layout section API now supports retrieving and removing sections by uuid.

Now that layout sections have uuid and weight, some API changes are done:

Before :

1. SectionListInterface::getSections() used to return a sequentially and numerically keyed array of section objects.
2. The methods SectionListInterface::getSection() and SectionListInterface::removeSection() had $delta as parameter.

After :

1. Added methods setUuid(), getUuid(), setWeight(), getWeight() to the Section class
2. Added new optional parameter to return the sections keyed by UUID inSectionListInterface::getSections().
3. Changed the methods SectionListInterface::getSection() and SectionListInterface::removeSection() to accept $uuid as parameter.