edje: add ability to reference images from other edje files

edje: add ability to reference images from other edje files

this uses the just-added "id" property to allow referencing images
by name from that theme. example:

=FILE1=
id: "myfile";
images.image: "someimage.png" COMP;

=FILE2=
requires: "myfile";
images.image: "someimage.png" EXTERNAL "myfile";

FILE2 will now load someimage.png from FILE1 at runtime if FILE1 is
currently opened in edje, and FILE1 will be kept open until FILE2 is
closed

@feature