ui: a press anywhere in the window closes the time-window popover

The window control is a GtkMenuButton whose popover autohides -- which works for as long as
GTK holds the popover's grab, and the grab is what can go missing: a window manager moving
it, the nested popover the unit dropdown opens taking it, and every headless run, where
there is no manager to hand one out at all. After that a click anywhere else in the window
leaves the bubble up, which is the "sometimes it is awkward to close" report.

`on_window_popover_press()` states the rule outright instead of relying on the grab: any
press that reaches the window's own surface closes it. That needs no hit test beyond the
button -- a popover is its own surface, so a click *inside* it never arrives here -- and the
button is left alone, because its own click is the toggle that closes it. The press gesture
is on the window in the capture phase, so a widget under the press (the plot's own gestures
among them) cannot hide it.

[windowpop] reads both halves, with autohide turned off for the probe -- the state a user
meets when the grab goes missing:

    [windowpop] opened=1 visible=1 click_gestures=2
    [windowpop] after a press elsewhere: active=0 (want 0)
    [windowpop] after a press on the button: active=1 (want 1)

`click_gestures` is read as well as the handler, so "the rule works" and "the rule is
connected" are two facts rather than one.

src/ui_app.hpp

@@ -1349,6 +1349,11 @@ gboolean zoomkeys_probe(gpointer user);
 void menus_probe_next(App& a, int step, guint delay_ms);
 void menu_row_labels(GtkWidget* box, GtkWidget** label, GtkWidget** accel);
 gboolean menus_probe_step(gpointer user);
+gboolean windowpop_probe_step(gpointer user);
+
+/// Closes the time-window popover when a press lands elsewhere in the window (see ui.cpp). Declared for
+/// the probe, which drives it and presses the button to check the one case that must *not* close it.
+void on_window_popover_press(GtkGestureClick* gesture, int n_press, double x, double y, gpointer user);
 gboolean color_probe(gpointer user);
 gboolean reseq_probe(gpointer user);
 void capture_probe_next(App& a, int step, guint delay_ms);