#90 MPA section remove image

[Steve Pugh]

In MultiPartArticles, each section can have its own
image. The problem is that once an image has been
added, it cannot be removed except by deleting the
whole section .

The following patch adds a "Delete Image" button which
enables images to be removed from a section, while
keeping the section content.

[Steve Pugh]

mpa delete image patch

[Alan Pevec]

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merged to trunk r1423