window: don't schedule a display of mnemonics if window has no modifier

window: don't schedule a display of mnemonics if window has no modifier

When the window has no mnemonics modifier set, as in the case of a
GtkMenu, never schedule a display of mnemonics on focus-in.

Previously, for those windows, the GdkModifierType mask fetched from the
device would typically have been zero, leading to the

mnemonic_modifier == (mask & gtk_accelerator_get_default_mod_mask ())

check to succeed, so we would always trigger a display for popup menus.

https://bugzilla.gnome.org/show_bug.cgi?id=697144