In the same way as also in GNOME 3.16 and in other desktop environments. The notifications can be sent via a notification daemon from the command line.
The command is part of the package libnotify-bin and is installed by the *-desktop meta-packages.
The dependencies are libc6, libglib2.0-0, libnotify4. Run strace to see the way of your notification:
% strace -e open notify-send foo bar
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libnotify.so.4", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libglib-2.0.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libgdk_pixbuf-2.0.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libgio-2.0.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libffi.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libpcre.so.3", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/x86_64-linux-gnu/libgmodule-2.0.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libz.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
To send your own notification
notify-send foo bar
From man notify-send:
SYNOPSIS
notify-send [OPTIONS] <summary> [body]
OPTIONS
-u, --urgency=LEVEL Specifies the urgency level (low, normal,
critical).
-t, --expire-time=TIME
The duration, in milliseconds, for the notification to appear
on screen. (Ubuntu's Notify OSD and GNOME Shell both ignore
this parameter.)
-i, --icon=ICON[,ICON...]
Specifies an icon filename or stock icon to display.
-c, --category=TYPE[,TYPE...]
Specifies the notification category.
Help options:
-?, --help
Show this help message
-h, --hint=TYPE:NAME:VALUE
Specifies basic extra data to pass. Valid types are int,
double, string and byte.
You can view the set keyboard shortcuts for Unity and Gnome through
gsettings list-recursively | awk '/hotkey/||/keybinding/||/media-key/' | less
and
dconf dump / | awk '/keybindings/ || /media-keys/{print; getline; print }'
I suggest you play around with those commands, try filtering out the output in different ways, but in short it's all in gsettings and dconf
Best Answer
Unfortunately, you'll have to compile your own version of the Gnome settings deamon to accomplish this: the key value is hard-coded in gsd-locate-pointer.c.
Alternatively, you could map an unused key or key combination to xdotool key 'Control_R'.