feature(macos): show dock icon when any app window is open (#4628)

On macOS with LSUIElement=true, Flameshot runs as an agent app (no dock
icon). This adds a dock icon that appears while any user-facing window
(Settings, About, Launcher, Upload History) is visible and disappears
when the last one is closed or hidden.

Uses QWindow::visibilityChanged signal with a per-window dynamic property
to track visibility state and a reference counter for the activation
policy toggle.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: borgmanJeremy <46930769+borgmanJeremy@users.noreply.github.com>