Add the ability to hide individual notes

[lukaszzalewski]

This lets you hide individual notes, as asked for in #78.

• Each note has a new hide button (sticky-hide, eye-slash icon) in its title bar, plus a Hide Note item in its context menu. The button uses the existing window-button CSS name and comes with its own SVG.
• The hidden state is saved with the note as a hidden key in notes.json. Notes saved without the key are treated as visible.
• Showing all notes (left-clicking the tray icon, --toggle, activating the app, clicking the group list in the manager) leaves individually hidden notes hidden. This is the problem raised about the close button in Add Close button and confirmation dialog to Sticky notes #147: there, a tray click brought back every note you'd closed.
• In the manager, hidden notes are dimmed, their titles are italic, and they show the XApp xsi-view-conceal-symbolic icon. The tooltip says the note is hidden. Clicking the icon shows the note again (the pointer turns into a hand over it), as does activating the note as usual, including from search results.
• Duplicating a note always creates a visible copy.

New translatable strings: "Hide Note" and "%s (hidden - click the icon to show)".

Screenshots

Testing

Tested on Linux Mint 22.3 (Cinnamon, X11) with a scripted GTK test that drives the real widgets, run against the branch as submitted:

• Hiding a note via its button or context menu unmaps it, and the change is saved to notes.json.
• A note saved as hidden stays unmapped on startup.
• Hiding all notes from the tray and showing them again restores the other notes but not the hidden ones.
• Activating a hidden note in the manager shows it and saves hidden: false.
• A single click on the hidden icon in the manager shows the note, and a click elsewhere on the preview does not. The pointer becomes a hand over the icon and resets when it moves off.
• Notes from an older notes.json without the hidden key can still be focused from the manager.

I've also installed the patched package on my machine and tried it by hand.