menubutton: Fix possible button/popover state inconsistences

menubutton: Fix possible button/popover state inconsistences

While a popover is hiding, the modal grab is already gone and the toggle
button is clickable again, but clicking again at that time will result in
gtk_widget_show() trying to show an already shown widget (although fading
out and hidden soon) and the toggle button activated.

So let the menubutton set the active status only if the menu/popover
widget wasn't already shown, and ensure this doesn't get triggered by
double/triple button press events.