editor for MANIFEST.MF or plugin.xml

The fix for bug 549818 broke the multi-page editors context menus in
case there are multiple editors trying to register popup menus with same
id's. The reason is that each PopupMenuExtender created for each page in
such an editor uses this.modelPart =
part.getSite().getService(MPart.class); which resolves to the *same*
model for *different* child editors. e4 layer doesn't provide different
models for child editors inside multi-page editor, so all child editors
re-use same "modelPart" object and so only the first editor will get
popup menus created.

To fix that, provide optional popup extender model id that will be used
to distinguish different menu contributions using same menu id. So every
child editor will get "his" dedicated "menuModel" and "his" popup menus
created.

Change-Id: Idaa26c5526e0e02296883f20655124c24a2c7129
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>