* widgets/modest-window.c: Add a method to get action from its private

* widgets/modest-window.c: Add a method to get action from its private
ui_manager instance. A new UI Dimming Manager was added too, and methods
for calling this dimming manager to check dimming rules.

* modest-mail-operation.c: Add a new private field into mail operation
to set an user defined callback to perform actions when some error
is detected during mail operation execution.
Add support to cancel tinymail operations, aborting camel operaiton
in progress. To identify operaiton to cancel, a new private field was
added to private section of mail_operation: account.

* modest-ui-dimming-manager.c: A new dimming rules design apporach.
very similar to UI Manager pattern.

pmo-trunk-r1988