Support notification actions for Ubuntu 22.04/Wayland

[RebeccaMahany]

When testing notification actions previously, @James-Pickett noticed that on his Linux laptop, notification actions would open the URL in a text editor instead of using the default browser. After more investigation, this issue appears to be present on Ubuntu 22.04, but not on the 22.04 VMs I have. My initial guess was that James's laptop uses Wayland and all of the VMs (accessed via VNC or RDP) are x11, but it could be some other difference between his environment and my VMs that causes the issue.

Fixes:

• First, default to x-www-browser over xdg-open, since x-www-browser does not appear to have the issue noted below figuring out what application to use to open the URL.
• Second, set DISPLAY for Wayland sessions as well. We cannot pull this from loginctl like we do for x11 sessions, so ultimately we look for the xwayland process, knowing that the display is the first command-line argument to this process. It's possibly a little brittle, but I couldn't find a better option.
• Finally, add the snapd directory where mimeinfo.cache lives to XDG_DATA_DIRS. If the user has installed their browsers only via snap (e.g. Firefox, even when installed via apt, actually installs via snap), then the mimeinfo defaults are stored in this separate directory that xdg-open cannot find, ultimately resulting in the behavior noted where the URL is opened in a text editor.

[directionless]

Seems okay. Though the labels imply there's some refactoring that might help.

[directionless]

Looks solid. But please put ctx first