Add a function to determine the text direction

This function can be used in the rare cases where the locale
has to be changed after gtk_init(). Based on a patch
by Jehan <jehan@girinstud.io>.

https://bugzilla.gnome.org/show_bug.cgi?id=720096
Disconnect signal handlers

We connect to notify::title on app-provided titlebars, but
we forgot to disconnect the signal handler when unsetting
the titlebar.
Sync the window title initially

When setting a custom titlebar that happens to be a GtkHeaderBar,
we connect to notify::title to pick up title changes on the headerbar,
but we forgot to sync the title initially. Fix that.
Update headerbar title from window title

This removes a possible source of confusion in our API.
The title is now synchronized both ways between the window
and the headerbar.

https://bugzilla.gnome.org/show_bug.cgi?id=720067
Fix a segfault in the event axes demo

Seems that pointer_info can be NULL, after all.
GtkHeaderBar: Add a ::has-subtitle property

It is a fairly common case to just want a title, and not
reserve extra space for a subtitle. This is much easier
to get right by setting a boolean property than by
constructing a custom title widget.
Use ::has-subtitle when constructing CSD titlebars

GtkWindow was constructing a custom title label to
avoid the size reservation for a subtitle. This is
now easier with the ::has-subtitle property.
Use ::has-subtitle in testtitlebar

This also gets simplified.
Trivial typo fix
Simplify a check

Whenever titlebar is set, title_box is pointing to the
same widget, so we can just always use title_box here.
Pointed out by Jasper St. Pierre.
Gettext comment for "default:LTR" must be before the string

https://bugzilla.gnome.org/show_bug.cgi?id=720096
Updated Galician translations
Updated Hebrew translation