fix event type on RootMouseDownHandler (#559)
fix event type on RootMouseDownHandler (#559)

This issue was addressed partially in the previous commit: 702f15052bbb124de2a2c9ede6a2dc436e470ffe, however, there's another type definition also have the same problem and been used on L126 and L130.

This PR fixes the remaining type errors.
cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown,cibernox/ember-basic-dropdown