libgui: 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.

This is a squashed commit based on an initial change with a couple of
fixes to avoid breaking certain targets. The source commits are:
  d723bd7669b4fc88dc282d8bf8ba5ecb2849d22f
  00d504c06ea034befe143e6b8cb34d004670ed02
  429ba89cd293633d2f882165066b422c96597df2

Change-Id: I034abec27bf4020d84af60d7acc1939c59986dd6
49 files changed