[Linux] Add IME Support for Wayland

[AiraYumi]

Description

Wayland is gradually becoming the default desktop environment, but in the case of SecondLife Viewer, it seems that the IME does not work on Wayland at the moment.

[Nicky-D]

The viewer is not Wayland ready at all and rather relies on XWayland

This might be why IME is broken on Wayland, but I'm not certain.

Wayland would also need a new fltk. I believe only 1.4 added Wayland support. And 1.4 isn't a general release yet, but rather is still pre release status.

[AiraYumi]

I tried replacing it with a build using the fltk 1.4 snapshot, but the viewer itself would not start.
Looks like more fixes are needed.

https://github.com/AiraYumi/3p-fltk/tree/testing/fltk_1.4
https://github.com/AiraYumi/3p-sdl2/tree/unrelease/enable_wayland_for_linux
https://github.com/AiraYumi/viewer/tree/unrelease/bump_fltk_1.4

[AiraYumi]

The viewer is not Wayland ready at all and rather relies on XWayland

Create a ticket regarding this matter.