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
Added attribute editor to the view menu.
Fixed QShortcut include.
Merge pull request #332 from est77/master

Added attribute editor to view menu. Fixed Qt include.
changed the type of the IOR inputs of the specular BTDF from text to numeric.
Merge branch 'master' of github.com:appleseedhq/appleseed
set a minimum width on the attribute editor dock widget.
translucency should be a color, not a scalar
Merge https://github.com/appleseedhq/appleseed
Merge pull request #337 from DaviesX/master

translucency should be a color, not a scalar
fixed bug in 2c0d047ec0e4e31061e441099d28911e6db72c75: wrong scaling by translucency factor.

minor code edits.
minor code cleanups.
reverted a2ac6285756bdc3073ddea410f9fb4e274c56d06.

Reverted a2ac6285756bdc3073ddea410f9fb4e274c56d06: delete widgets immediately in appleseed::studio::clear_layout(). The reason is that we often have signals in flight when we clear layouts (for instance when we rebuild the entity editor form when changing the model of a BSDF), so deleting widgets immediately is incorrect and leads to crashes.
A possible fix for possible clang alignment issues.
Merge https://github.com/appleseedhq/appleseed
Reuse foundation pow2 functions.
Add ability to assign materials to both sides of an object instance at once