Add USR1 signal handlers to show widget menus

[apebl]

This allows the menu to be displayed by sending a SIGUSR1 signal to a widget process.

Why

GNOME Shell does not support legacy tray icons, so I created a wrapper extension for use on GNOME. This extension simply sends a USR1 signal to widgets to open the menu.

https://github.com/apebl/gnome-ext-qubes-tray-widgets

Why not using tray icon extensions

The tray icon extensions I tested lead to shell crashes when a VM tries to create tray icons:

• https://github.com/ubuntu/gnome-shell-extension-appindicator
• https://github.com/martinpl/tray-icons-reloaded

It seems that this issue is not specific to an extension but rather an incompatibility between GNOME and Qubes OS.

Instead of bothering with the problem, I decided to implement a simple alternative method as I described.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.71%. Comparing base (5902fa1) to head (c4002a0).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   93.72%   93.71%   -0.01%     
==========================================
  Files          57       57              
  Lines       10843    10843              
==========================================
- Hits        10163    10162       -1     
- Misses        680      681       +1     

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[marmarta]

This is a problem with Gnome, yeah; they decided nobody actually needs tray icons, basically, and so they will not support them fully, except for Gnome's own.

Anyway, on-topic: I generally like this, just one ask: could you document it also? In the code comments, but ideally also in qubes docs themselves? So that it's easier to find for someone who is not you :)