[Merge] ~binli/ubuntu/+source/gdm3:ubuntu/bionic into ~ubuntu-desktop/ubuntu/+source/gdm3:ubuntu/bionic

[Bin Li]

@Iain,

 I just found in bionic we also missed the gdm-disable-wayland binary file, I just cherry-picked from cosmic. Sorry about it. Thanks!
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https://code.launchpad.net/~binli/ubuntu/+source/gdm3/+git/gdm3/+merge/356297
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