popup: remove previous object when setting new content

popup: remove previous object when setting new content

Summary:
Other widgets remove previous object when setting new content, but popup does
nothing for the part whose name is "default".
content_unset can be used to keep previous content object, so content_set
(especially with NULL) should be able to delete previous content object
for uniformity of the entire widgets.

@fix

Reviewers: jpeg, singh.amitesh, cedric, raster, conr2d

Reviewed By: conr2d

Subscribers: minkyu, herb

Differential Revision: https://phab.enlightenment.org/D4885