Add rebase button to the change screen

This change adds a rebase button along with the rest of
the action buttons in the change page. When pressing the
button, the most recent patch set will be rebased onto
the tip of the destination branch or the latest patchset
of the change we depend upon. A new patch set containing
the rebased commit will be produced and added to the
change.

Content-merging is always used, regardless of the project
setting for use of content merges.

This change started long ago as a draft by Magnus Bäck
and the very nifty rebase trick (rebasing without the
need for a working index) is his.

Since then a lot has happen and I would like to discuss a
bit around the design choices I made along the way:

Design choices
--------------
* Currently all users that can upload new patch sets to a
change are also allowed to see the rebase-button.

In the future, I plan to add a rebase capability to Gerrit,
so the right to rebase other users' changes could be limited
to include only certain powerusers (depending on project and
refs).

I prepared by adding the canRebase() to ChangeDetail and
ChangeControl instead of just linking directly to
canAddPatchSet().

* The emailing is subclassed from ReplacePatchSetSender
which I think makes sense.

* I lifted some code from ReceiveCommits, which allows
me to move votes to new patch sets, to ApprovalsUtil.java.
Some refactoring could be made in ReceiveCommits to make
use of that code, but I would prefer to do it in a later
and separate change due to the importance and complexity
of ReceiveCommits.

* As long as user has the correct access rights to execute
a rebase within Gerrit, the button will be visible and
enabled. I chose not to do any automagic merge-testing
before showing the actual Rebase button. This approach was
chosen since it contributes to cleaner code and better
performance. We do not really want to execute merge tests
just because the user is looking on the change-screen,
especially since a rebase operation is not issued that
often.

* Of course, the rebase can fail for various reasons, and
there are decent error messages explaining why the rebase
was not allowed to be executed.

* The new GerritWidgetCallback was introduced. Parts in
gerrit-gwtui can and should be refactored to make use of
this callback.

Bug: issue 1035
Change-Id: I7eba96dfa769690eddce43fe9ea3c6cd39fe01a0

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeScreen.java

@@ -321,6 +321,8 @@ public class ChangeScreen extends Screen
     }
 
     dependenciesPanel.setOpen(depsOpen);
+
+    dependenciesPanel.getHeader().clear();
     if (outdated > 0) {
       dependenciesPanel.getHeader().add(new InlineLabel(
         Util.M.outdatedHeader(outdated)));