fix dropdown: trasnclude to select toggle's scope

Apply the same sort of fix to the dropdown's toggle than to the
dropdown's menu. By using a "ng-transclude 1", the scope inside the
toggle is the same as the DOM where the dropdown is created.
fix select: scope management after dropdown fix

The scope is better managed inside the dropdown directive. The select
directive uses the dropdown directive. So by changing the way the scopes
are managed in the dropdown, a cleaner version is now done inside the
select directive.