Warn the user when available RAM / memory is low

Originally created by @emmapeel on #7782 (Redmine)

From a user point of view, being warned when the computer is running low of RAM might be useful to understand why some software crashes or the computer gets slower or freezes, and take the needed action to remove stuff or close programs.

Marking this as a research ticket for the moment.

What others do

• Fedora enabled earlyoom as a short-term measure. Longer-term, they want to consider https://gitlab.freedesktop.org/hadess/low-memory-monitor/.
• Fedora enabled zram (#5740 (closed))
• There's also systemd-oomd, but, according to the man page it does not work well without swap ("Without swap, the system enters a livelocked state much more quickly and may prevent systemd-oomd from responding in a reasonable amount of time"): #19454