As gtk_adjustment_new() returns a GtkAdjustment* now

https://bugzilla.gnome.org/show_bug.cgi?id=630731

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=630731

gtk_widget_set_visual() exists now and can be used instead.

This reverts commit 59b227e1.

It turns out that the previous handling of just providing a way to set
visuals just on toplevels was not sufficient. In particular it
complicated the various implementations of the tray icon specification.
This patch reintroduces gtk_widget_set_visual() which behaves very
similar to GTK2's gtk_widget_set_colormap().

A future commit will remove the gtk_window_set_visual() function.

Also remove screen parameter from _gdk_offscreen_window_new() and get
rid of a bunch on includes in gdkoffscreenwindow.c

gtkimcontextsimpleseqs.h is not really autogenerated but shipped

https://bugzilla.gnome.org/show_bug.cgi?id=630708

gtk_paint_layout is utterly broken. Someone needs to fix it so we use
the cairo_t's matrix and don't juggle with both Pango and cairo matrices
everywhere.

Fix compile warning and use cairo_paint() instead of querying the widget
size and cairo_fill()ing it.

With recent changes, nobody compiled with debug enabled. Fix that.

When fitting a widget into its allocation, the second dimension
is always dependent on the first, so gtk_widget_get_preferred_size()
cannot be used directly (because we want the natural height for
the allocated width, not the natural height for the natural width,
which is generally a smaller height than the height-for-minimum-width
or height-for-allocated-width).

Added test to testadjustsize to ensure proper behaviour.

Also make GtkWidget derive from GInitiallyUnowned