Rewrite UI plugin action description API

Using methods on an interface that plugins/extensions implement
makes for a very brittle API. We cannot safely add a new method
without breaking all existing code or introducing another interface
for plugins to implement.

Change the UI action description to be a single method interface
that returns an instance of a concrete description object. Plugins
can populate the fields they know about and need for their use case,
and the server will do its best to honor what the plugin has given.
New methods can be added to the description without affecting the
API versioning, enabling Gerrit to add more features in the future.

The confirmation message feature has been removed. In the near
future plugins will have the ability to register JavaScript code
to be executed client-side when the button is activated and before
the RPC is started. This will allow plugins to perform more custom
logic, such as obtaining additional input.

Looking even further ahead plugins may register their own JS handler
to render a button, allowing the plugin to use a more complex widget.
The UiAction.Description type may need to be extended to allow
server side view code to pass custom data to the UI rendering logic.

Change-Id: If5e0e6b42d6d1e8a111a2d304ce54b9d8834688a

gerrit-common/src/main/java/com/google/gerrit/common/data/UiCommandDetail.java

@@ -21,5 +21,4 @@ public class UiCommandDetail {
   public String label;
   public String title;
   public boolean enabled;
-  public String confirmationMessage;
 }