Support falling back to X11 from Wayland

[koonix]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Adding Wayland support (-tags wayland) removes X11 support.

Is it possible to construct a solution with the existing API?

No response

Describe the solution you'd like to see.

Fyne apps built with Wayland support should try Wayland but fall back to X11 if Wayland is unavailable, so that a single binary is sufficient for one platform, regardless of the available display server.

[Jacalz]

Glfw v3.3 only has support for building against one display protocol at a time, not both, so we can't really do anything about this until we update to v3.4. The plan is to build for both X11 and Wayland by default once go-gl/glfw#400 is done and likely have -tags wayland be Wayland-only with an x11 tag for the opposite.