configure: Move AC_USE_SYSTEM_EXTENSIONS up

We don't want to get warnings from autoconf about
AC_USE_SYSTEM_EXTENSIONS coming after AC_COMPILE_IFELSE.
Use CHECKED instead of ACTIVE for expanded expanders

https://bugzilla.gnome.org/show_bug.cgi?id=733967
Adwaita: Make expanders turn again

The theme needed some small adjustments for the recent expander
state changes ACTIVE to CHECKED.
theme: Specify the weight, not the font, for header bar titles

Using `font: Bold` will override the font with whatever the system
resolves a `Bold` font name to, including the font's own default size.

We want the font weight to be bold, but we want the default font name
and size to come from the GtkSettings:font-name property, like we do for
the rest of the system.
Adwaita: rationalize font sizes.

...making GtkSwitch font-size relative in the process.
Adwaita: take named colors out of _colors.scss

To be able to use sass for shipped css, importing _colors.scss
and _drawings.scss.
css: Implement font-stretch

The font-stretch CSS property is defined in the Level 3 CSS Fonts
module, available at:

  http://dev.w3.org/csswg/css-fonts/#propdef-font-stretch

It allows defining a normal, condensed, or expanded face to the font
description. Pango already supports it, so this is literally just the
CSS parser machinery needed to bridge our CSS to the FontDescription
API.

https://bugzilla.gnome.org/show_bug.cgi?id=735593
Adwaita: border rationalization and cleanup

Rely only on border-style for border, take out border-width and
border-style from drawing functions.
tooltip: Use subsurfaces on wayland

This places tooltips in the right place, and avoids having those attempt
to steal the focus.

https://bugzilla.gnome.org/show_bug.cgi?id=735531
scrolledwindow: Be smarter at invalidating overshoot areas

The previous way to invalidate was meant to work on the overshoot window
so it wouldn't be as taxing. Since the overshoot window is gone, this
would invalidate way more than intended. So constrain invalidated areas
to the sides where overshoot is happenning at that moment.

https://bugzilla.gnome.org/show_bug.cgi?id=735223
Updated Hebrew properties translation
Adwaita: nicer transition for flat buttons.
quartz: Get default font size from user font

https://bugzilla.gnome.org/show_bug.cgi?id=735321
quartz menu: update visibility property name

This property is called "is-visible" now, not "visible".

https://bugzilla.gnome.org/show_bug.cgi?id=735122
GtkMenuTrackerItem: fix submenu visibility flag

We were only properly setting the "is-visible" flag to TRUE for menu
items with associated actions and not (for example) on submenus.

This was fine because the code for building GtkMenus from models
(correctly) assumed that submenus should always be visible and never
checked the property.

This is not true for the Mac OS code, which actually checked the
property and found it to be false for submenus.

Initialise the property to TRUE so that we get the correct value
reported for items that don't have actions.

https://bugzilla.gnome.org/show_bug.cgi?id=735122
GtkMenuTracker: one more visibility tweak

On creation, we call action_removed() in case the action was missing
from the start.  Because we just created the action, 'can_activate' will
always be FALSE here and this function will therefore always do nothing.

We do want the visibility state to be updated though, for the case where
the action is missing but the item should still be visible from the
start.

Update the visibility directly instead of trying to call
action_removed().

https://bugzilla.gnome.org/show_bug.cgi?id=735122
gtksettings: Set default antialias to CAIRO_ANTIALIAS_DEFAULT
application-quartz: Implement openFiles delegate

This adds a straightforward implementation of the openFiles app
delegate method. The call is translated to a call to g_application_open
if opening files is supported by the application.

https://bugzilla.gnome.org/show_bug.cgi?id=722476
showuri: forcefully use [NSWorkspace openURL] on OS X

This is of course not the correct way to fix this, which would instead
be to implement a proper GAppInfo backend for OS X. This will hopefully
land soon, but in the mean time this hack is better than the alternative.