Standardizes dialogs, preference page and content-type tree. 

Fixed a potential exception reproducible by selecting an unlocked file
association (Edit button enabled), then selecting a different content
type, then clicking the Edit button.

Change-Id: Ib362ac366a847f485c250a7707f74e90556812aa
Signed-off-by: Sopot Cela <scela@redhat.com>