Run clang-tidy modernize-use-override to remove all the redundant
virtual keywords from overridden methods, and add some missing
overrides.
Another benefit is that this has also added the overrides
on destructors, which will cause a build failure if a base
class is missing a virtual destructor.
and new widget QgsComposerItemComboBox for showing matching composer
items.
Swap existing comboboxes to use the new widget, which removes a lot
of fragile code designed to allow selection of items. Additionally
the combobox now show the correct item id rather than always showing
"Map 0/1/..."
