Build with "-Werror" on all platforms

[rgorosito]

• Seen in version 20180729:

The flag -Werror only was enabled on Linux, but it broken the build with modern versions of gcc. To solve this issue, is required to update de Qt version an fix all warnings

[tracernz]

Yes. I think the best course would be to move to Qt 5.12.2 (current LTS). There is a bit of background work that needs to happen for that to occur though. Maybe it's better to disable Werror for now?