Linux: Allows GUI to launch in a Wayland-only environment (#1264)

Currently we check whether the system has DISPLAY environment variable
set, which is the case in a system that uses X11 natively or XWayland.
This variable is not set in a system with only Wayland, so we need to
also check whether WAYLAND_DISPLAY is set.

Fixes: #184

src/Main/Unix/Main.cpp

@@ -76,7 +76,7 @@ int main (int argc, char **argv)
 #endif
 
 #ifdef __WXGTK__
-		if (!getenv ("DISPLAY"))
+		if (!getenv ("DISPLAY") && !getenv ("WAYLAND_DISPLAY"))
 			forceTextUI = true;
 #endif