UML-195: Fix a wrong dependencies to uml2 and cleanup images

Conflicts:
	plugins/org.obeonetwork.dsl.uml2.design/images/required-bottom.svg
	plugins/org.obeonetwork.dsl.uml2.design/images/required-left.svg
	plugins/org.obeonetwork.dsl.uml2.design/images/required-right.svg
	plugins/org.obeonetwork.dsl.uml2.design/images/required-top.svg
	plugins/org.obeonetwork.dsl.uml2.design/plugin.xml
	plugins/org.obeonetwork.gmf.runtime.diagram.ui.extension/META-INF/MANIFEST.MF