This is the filename convention you get when you define a shared module
in meson, and since there is no particular reason to not include the
"lib" prefix, lets make it easier to port it over. While at it,
de-duplicate the retrieval of the plugin name.
- Moved xdisplay, name and various atoms from MetaDisplay
- Moved xroot, screen_name, default_depth and default_xvisual
from MetaScreen
- Moved some X11 specific functions from screen.c and display.c
to meta-x11-display.c
https://bugzilla.gnome.org/show_bug.cgi?id=759538
