widget: Ignore text direction in gtk_widget_set_state_flags()

Previously we did a semi-successful job at ignoring it. Unfortunately
this job was bad enough that we could lose the direction.

We still allow passing in the enum values, because we want code like
this to work:
  set_state_flags (get_state_flags() | SOME_FLAGS)
docs: Use markup for links
docs: remove <emphasis>
Add GTK_STYLE_CLASS_SUGGESTED_ACTION
Add GTK_STYLE_CLASS_DESTRUCTIVE_ACTION
popover: Listen on grab_notify from the relative-to widget

Anytime ::grab-notify comes across, the popover visibility and GTK+
grab ownership are checked, so the popover is hidden when it loses
the GTK+ by any reason.
popover: propagate (insensitive|backdrop) flags from relative-to widget

Popovers aren't direct children of the widget they point to, but yet
they act as children of it, so do the same with state propagation,
so the flags that propagate across the hierarchy reach popovers too.
docs: use links instead of <xref>
docs: use proper apostrophe

https://wiki.gnome.org/Design/OS/Typography
docs: use more apostrophes
docs: use apostrophes in *n't
docs: use apostrophe in *'ll
docs: use apostrophe in *'re
docs: use apostrophe in *'ve
docs: use apostrophe in *'d
docs: use apostrophe in *'m
docs: use proper quotations instead of '*'
Introduce GtkModelButton

Similar in spirit to GtkModelMenuItem, this private GtkButton subclass
can connect to a GtkMenuTrackerItem and present itself as either a
regular button, a check button, or a radio button. Activation and
state tracking is done through the GAction that is associated with
the menu tracker item.

https://bugzilla.gnome.org/show_bug.cgi?id=723014
Populate popovers from menu models

This adds a new function, gtk_popover_new_from_model, which creates
a popover and populates it with suitable content according to the
menu model. The current implementation uses GtkModelButton for the
individual items, and a GtkStack for submenus.

https://bugzilla.gnome.org/show_bug.cgi?id=723014
Add a test for model-based popovers

https://bugzilla.gnome.org/show_bug.cgi?id=723014