DevMode: wait for CodeServer to start before enabling buttons

This is to give CodeServer time to write the launcher files,
so the user doesn't see a blank page.

Bug: issue 8956
Change-Id: I6966352652e4bcf83bf0bc39439eb245d7e9635a
Review-Link: https://gwt-review.googlesource.com/#/c/10121/
(cherry picked from commit f0f5a02bb976a31b489988bd857b2726abd43e2d)