Sync with internal BUILD changes.

Change-Id: I6d6c2bcfc5a86ea9d610d7a9df56ed38f039fc1d
Fix attach / detach handling in Composite and CellTable

1. Make Composite calls doAttachChildren() and doDetachChildren() while
handling attach and detach events. This is necessary because a few
subclasses like CellTable have children widgets that are not under the
wrapped widget's hierarchy.

2. Attach and detach messagesPanel in doAttachChildren() and
doDetachChildren() of CellTable, just like what CellList does, so that
event handling of widgets under empty table widget's hierarchy is
working properly.

Change-Id: I5572e3ed6e7d53e8e4c5dc3d00dfd6ee0efa62fb
Review-Link: https://gwt-review.googlesource.com/#/c/5565/
Merge remote-tracking branch 'upstream/master'