To make it happen <collapsible-group> directive had to undergo the
same transformation that was applied to <panel> directive: instead of
passing just a '@title' reference, the whole '=content' reference is
now passed to the <collapsible-group>'s scope. This allows to use
<editable> directive inside it with 'ng-model=group.title' as
getter/setter method. Yet <collapsible-group>'s scope wasn't remade as
radically as <panel>'s was - '&on-add' and '&on-remove' are still
there and left for future refactoring.
Change-Id: I4de7a542f282efee6deb34d4957a5873d617ad64
Closes-Bug: #1446171
Closes-Bug: #1446226
531dc56c64