When running on Wayland, gdk_display_get_name() can return things like
"wayland-0" rather than valid X display names. PuTTY nonetheless
treated them as X display names, meaning that when running under
Wayland, pterm would set DISPLAY to "wayland-0" in subprocesses, and
PuTTY's X forwarding wouldn't work properly.
To fix this, places that call gdk_display_get_name() now only do so on
displays for which GDK_IS_X_DISPLAY() is true. As with
GDK_IS_X_WINDOW(), this requires some backward-compatibility for GDK
versions where everything is implicitly running on X.
To make this work usefully, pterm now also won't unset DISPLAY if it
can't get an X display name and instead will pass through whatever value
of DISPLAY it received. I think that's better behaviour anyway.
There are two separate parts of PuTTY that call gdk_display_get_name().
platform_get_x_display() in unix/putty.c is used for X forwarding, while
gtk_seat_get_x_display() in unix/window.c is used used for setting DISPLAY
and recording in utmp. I've updated both of them.
Correcting a source file name in the docs just now reminded me that
I've seen a lot of outdated source file names elsewhere in the code,
due to all the reorganisation since we moved to cmake. Here's a giant
pass of trying to make them all accurate again.
This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.
