This creates efl_ui.eot
It's not called efl_ui_types.eot because a file with that name already
exists in efl/interfaces (for Efl.Ui.Drag functions).
Also add some FIXME comments, and move some types to elm_widget_item.eo.
Ref T5329
This is a new type representing a mutable string (no const).
Regular strings cannot be made mutable with @owned because
they might be hidden behind typedefs.