Fix :visible updates being lost for unmapped widgets

[61021]

Description

Fixes #1451.

The initial value of :visible is applied at the widget's first map (connect_first_map) so that it survives the window's show_all — but the value was captured at build time. For widgets whose first map happens later (children of a closed revealer being the common case), every reactive :visible update issued in between was clobbered by that stale snapshot the moment the widget finally mapped: update vis=true while the revealer was closed left the widget hidden after revealing, even though eww state showed the right value.

The fix keeps the latest value in a shared Rc<Cell<bool>>: the reactive visible prop writes to it, and the first-map handler reads it instead of a build-time copy. No behavior change for the ordinary cases (initial value application and reactive toggling while mapped work exactly as before — the cell starts with the evaluated initial value).

Usage

No configuration changes — existing configs just start behaving as expected:

(defvar reveal false)
(defvar vis false)

(revealer :reveal reveal
  (label :visible vis :text "hello"))

$ eww update vis=true     ; while the revealer is closed
$ eww update reveal=true  ; label now correctly appears

Showcase

Same sequence (vis=true while closed → reveal), before and after, on master 48f5aa8:

before (bug: vis: true but widget hidden)	after

The hide-while-closed direction (which already worked, since a hidden widget never maps) is unchanged:

Additional Notes

• Verified on a live Wayland/Hyprland session (both directions, plus toggling while revealed).
• cargo fmt and cargo clippy clean; CHANGELOG entry added.

[61021]

Bump 🙂 — fixes :visible updates being silently lost for unmapped widgets. Happy to rebase if anything's drifted.