add multi-view file chooser
change FileChooserLayout handling
restrict layout property to FileChooserLayout subclasses
Merge pull request #2525 from kived/filechooser-multiview

add multi-view file chooser
add versionadded/versionchanged markers
Merge pull request #2577 from kived/filechooser-multiview

add versionadded/versionchanged markers
fix position of file chooser layouts
Merge pull request #2578 from kived/filechooser-multiview

fix position of file chooser layouts
py3 compatibility fix for osx packaging

Fixes https://github.com/kivy/kivy/issues/2398.
update add_widget exception message