fixed regression introduced in b310276f19f4be0f808120d8ee88c7c8d567943b ("fixed Qt runtime warning").

QLayout::isEmpty() returns true even if the layout contains items:
https://qt.gitorious.org/qt/qt/source/0315971ee951e9abe7f288564ddf2e81aeed1fd8:src/gui/kernel/qlayout.cpp#L609
delete widgets immediately in appleseed::studio::clear_layout().
added an attribute editor to appleseed.studio.

known issues:
- the initial width of the attribute editor is too small.
- when editing an entity through the entity editor dialog (by double-clicking on the entity), the attribute editor is not updated with the new values after the entity editor dialog is closed.
Merge branch 'master' of github.com:appleseedhq/appleseed