pathbar: add back GTK_STYLE_CLASS_LINKED

This was accidentally removed in commit
42f53ab58e3f905b242029c2ea446d79d2113725

https://bugzilla.gnome.org/show_bug.cgi?id=706028
themingengine: refactor a conditional branch

Return early when the state is not wildcarded - makes the code more
readable.

https://bugzilla.gnome.org/show_bug.cgi?id=705443
css: Add a GtkCssValue for GtkCssImageEffect

This will allow us to use the CSS machinery to apply a highlight or dim
effect when images are prelit or insensitive.

https://bugzilla.gnome.org/show_bug.cgi?id=705443
themingengine: apply image effects according to GtkCssImageEffect value

Instead of only looking at the state flags.

https://bugzilla.gnome.org/show_bug.cgi?id=705443
iconfactory: cache/invalidate pixbufs according to the effect

The state of the widget is not enough now to cache the pixbuf - we also
have to take into consideration the image effect itself, since the state
on the actual GtkStyleContext we use might not change, e.g. because the
change was on a parent context.

https://bugzilla.gnome.org/show_bug.cgi?id=705443
iconhelper: refactor some code into a single function

https://bugzilla.gnome.org/show_bug.cgi?id=705443
Avoid deprecation warnings
Enforce a fixed scale of 1 if no cairo scale support

We used to just follow the default, which would pick up scale
changes from xsettings, but that is not right if we can't
actually support scales.
x11: Add gdk_x11_display_set_window_scale

This lets you force a specific window scale, this is needed
for mutter to be able to disable the scaling as it needs access
to unmangled X window/screen sizes. It can also be useful to
force a specific scale in e.g. tests.
Update README.in
clipboard: Split struct definitions into private header
clipboard: Make a bunch of functions vfuncs
clipboard: Reimplement Wayland clipboard using vfuncs

This way, the Wayland and the regular clipboard implementation can both
be compiled in and selected based on the display in use.

One thing potentially broken now is text mime type handling as Wayland
seemed to use different mime types in some places.