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:
  d723bd76
  00d504c0
  429ba89c

Change-Id: I034abec27bf4020d84af60d7acc1939c59986dd6