css: Add a :checked pseudoclass

https://bugzilla.gnome.org/show_bug.cgi?id=733967
gtk: Use new :checked state

on:
- GtkToggleButton
- GtkCheckButton
- GtkRadioButton
- GtkModelButton
- GtkCellRendererToggle
- GtkCheckMenuItem

also update themes:
- Adwaita
- Raleigh
but not the win32 theme.

The new :checked state replaces :active for the actual checkedness of
the widgets and :active is now used exclusively while the button is being
pressed.

https://bugzilla.gnome.org/show_bug.cgi?id=733967
button: Don't honor child-displacement anymore

It doesn't make sense to support child displacement in a world where
pseudoclasses behave different from the actual displacement states.
Nobody would ever understand why a widget is displaced.

It is easily possible to simulate child displacement by using padding
CSS properties.
button: Remove depressed variable

It is not used anymore.
css: Do inconsistent checked states right

In HTML5, both pseudoclasses apply. So we do the same thing in our
widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=733967
button: Remove depressed_on_activate

All buttons should always be marked as :active when they are pressed.
That includes checkboxes (which are never activated in real code anyway,
so this change pretty much doesn't matter).
gtk: Don't use gtk_render_activity()

... in places where we draw a background. This was changed for GTK 3.0.0
to allow animations, but these days it doesn't make sense anymore to use
gtk_render_activity() for backgrounds.
stylecontext: Update docs for gtk_render_activity()
themingengine: Remove hack to draw backgrounds instead of checks

This was a hack we added in early 3.x to allow themes to customize their
checkmarks.
Now that we want to properly support real backgrounds everywhere,
supporting this feature would cause double draws of backgrounds.
widget: Add checks for setting state flags

This is more for GTK developers to catch when they forgot to change
GTK_STATE_FLAGS_BITS after adding a new state flag than to prevent
widget developers from using the wrong flags.
button: Handle :checked in gtktogglebutton.c

Don't try to handle it elsewhere.
button: Handle :inconsistent in GtkToggleButton
radiobutton: :hover on hover

No special cases, those go in the theme.
togglebutton: Don't duplicate button code

It's the same code, so no need to write the same vfunc twice.
togglebutton: Remove unneeded code

The state is completely handled by gtkbutton.c so no need to do anything
in the togglebutton.
Fix make install

The path to gtk-update-icon-cache was set incorrectly in one case.
Update expected output in a11y tests for buttons

This is an expected change from the recent button state cleanups.
Update expected output for the font picker a11y test

This change is a consequence of the recent font chooser changes.
More updates
stylecontext: Move private a11y function to private header