Improve keyboard navigation and annotations throughout the app for better accessibility (a11y)

GNOME Calendar is not known to be very accessible nor keyboard-friendly. This is a partial list of currently known 8. Accessibility issues we have been able to find so far. We are not a11y experts, so help is welcome to

• Figure out how visually-impaired people use calendaring apps, and what they would expect in terms of general UX
• Figure out what exactly is missing or broken, technically
• Implement fixes for anything below and 8. Accessibility issues.

Observed problems

Jeff's (non-expert) casual observations on GNOME Calendar 44.x's keyboard navigability:

• There seem to be some keyboard navigation issues with the mini calendar gtk widget:
  • You can't focus the < > arrows
    • #933 (comment 1773532) might incidentally solve one of those issues if we rip those < > buttons back into the headerbar
  • Focus goes directly to individual days (no way to focus/skip the whole grid block thingy first? should there be?)
• You can focus events in the sidebar but not activate/open/edit them in any way; it might be that the focus is on the wrong widget (i.e.: the row instead of the child item within the row)... but we need someone to figure out how to solve that.
• Once you run through the events/widgets of the sidebar, focus then shifts to the headerbar... but after the headerbar there is no continuation to the main view, it loops back to the UI's beginning.
  • The week and month views' canvas/widgets are a black box, and non-interactive (like the sidebar in 43-44.x)
  • It is unclear to us whether the month view and week view should even be made accessible (see #1061 (comment 1919276)), vs just the sidebar
• In the event editor dialog, the "notes" textfield eats Tab and Shift+Tab so if you ever enter that field you are forever trapped. Unless indenting/unindenting a markdown list content line, there is no reason to have tabs be parsed as text there. !344 (merged)

Development tasks

Appetizer:

• Review MR !344 (merged)
• Review MR !252 (closed)
• Review issue #524 (closed)
• Turn on the accessibility warnings in the GTK Inspector and see if the list below needs to be expanded (see also this blog post)
• Review MR !348 (closed)

"Low-hanging fruit" problems to fix:

• Inconsistent behaviour in the sidebar calendar widget
  • Keyboard navigation within the minicalendar should use arrow keys, not Tab (Tab should directly jump to the events listview below it)
  • Focus doesn't work with the sidebar listbox unless you press Tab while the last day of the minicalendar is selected
  • Impossible to activate events in the agenda (items in the sidebar listview)
    While you can select items (events), you cannot enter their detailed view nor edit them
• Focus goes "poof" when leaving the sidebar
• Ensure that all icon-only buttons have a11y labels (tooltips alone are not enough, they should only be used as a fallback)

Harder problems to fix (how much effort would that be, considering issue #603 might be in the plans?) :

• Figure out what the planned new FreeDesktop accessibility architecture means for us in practice as a GTK4 application, once it becomes available someday
• If week view and month view need to be accessible (see #1061 (comment 1919276)), fix them:
  • Week view doesn't implement focus handling
  • Month view doesn't implement focus handling, and has no keyboard navigation/operations: #1061