libui: Enable -Weverything and -Werror

Enables -Weverything and -Werror, with just a few exceptions for
warnings we can't (or shouldn't need to) work around.

Change-Id: I470a267e9b1c373f2d5c960f005c3b0e02b2db63
(cherry picked from commit 303b9a51239d36d237a7d40c67b5085cdb3c1059)
15 files changed