CORE-4126 Added MVP plumbing to DEView and implementation.

Added showSystemNotification methods to the DEView and
DEView.DePresenter interfaces and their respective implementation
classes.
CORE-4170 Created Announcer widget that displays announcements.

This version is not stylish.
IN PROGRESS - issue CORE-4126: Add system notification display area to
DE. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4126

Added plumbing for the system messages window
Merge branch 'gxt3-sandbox' of https://github.com/iPlantCollaborativeOpenSource/de-webapp.git into gxt3-sandbox

Conflicts:
	src/main/java/org/iplantc/de/client/desktop/views/DEViewImpl.java
IN PROGRESS - issue CORE-4126: Add system notification display area to
DE. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4126

The system messages window now always displays the no messages available
message.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Added a system messages viewing window that wasn't styled and isn't
backed by real data.
Merge branch 'gxt3-sandbox' of https://github.com/iPlantCollaborativeOpenSource/de-webapp.git into gxt3-sandbox
Merge branch 'gxt3-sandbox' of https://github.com/iPlantCollaborativeOpenSource/de-webapp into gxt3-sandbox
Merge branch 'gxt3-sandbox' of https://github.com/iPlantCollaborativeOpenSource/de-webapp into gxt3-sandbox
CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Refactored MessagePoller so that it can support polling for multiple
items.
CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Refactored message polling. The MessagePoller class no longer knows
which tasks it is running. The DEPresenter adds the tasks to the
MessagePoller on initialization.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Created event used to indicate that one or more new system messages are
available for the user to read.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Created the data model.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Moved the system message related services into the
org.iplantc.de.client.sysmsgs.services package.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

added TODO to IdListDTO
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Eliminated SystemMessage, replacing it with MessageDTO.
Created a local cache of the local system messages.
Switched the presenter to using the cache.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Moved the SystemMessageCache class into its own package.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

Trimmed the DTO suffix off of the model classes.
OPEN - issue CORE-4122: User can view system messages. 
https://pods.iplantcollaborative.org/jira/browse/CORE-4122

The view can now display a spinner to indicate that it is loading system
messages.
CORE-4126 Add system message display window

Created a cell version of the summary list element view.