fix(gtk/toggle_fullscreen): header bar visibility

[pseudocc]

While unfullscreen the window, header.setVisible(true) will be called. For .gtk it's okay, because it will follows the window-decoration, while .adw doesn't.

After #4850, the header is always created and added to the window, so setting visibility to the correct value is vital for .adw.

Fixes: #4850

[jcollie]

Unfortunately, the interaction between window-decoration, gtk-titlebar and the various options for toggling window state are poorly specified. Because my reading of the documentation comments is different, and it could be seen as a bug that the titlebar doesn't reappear after toggling fullscreen. We need to have a larger discussion on these options and come up with ways that users can clearly express their intentions for window decoration and titlebars. Until that happens, any PRs that change the behavior are just going to cause additional churn when people with differing interpretations report bugs or submit PRs.

[pseudocc]

Unfortunately, the interaction between window-decoration, gtk-titlebar and the various options for toggling window state are poorly specified.

• True.

@jcollie But I am not proposing a behavior change, I am trying to keep it as it was (before the previous PR), the previous PR makes gtk and adw behave differently.

Expected and actual

window-decoration=false, gtk-adwaita=true

steps	has titlebar
1. launch ghostty (non-fullscreened)	no
2. fullscreen	no
3. unfullscreen	yes(before the previos PR, no)

window-decoration=false, gtk-adwaita=false

steps	has titlebar
1. launch ghostty (non-fullscreened)	no
2. fullscreen	no
3. unfullscreen	no