Since GTK 3.8 gtk_widget_register_window() should be used instead of
gdk_window_set_user_data() in a widget's ::realize() method.

gtk_widget_unregister_window() is called by the GtkWidget's
::unrealize() handler we chain up to.

This has no functional impact.

It also drops use of gtk_style_context_get_background_color() that has
been deprecated in 3.16.

This however has no functional changes.

GtkScrolledWindow in GTK 3.16 might have a minimum and natural height
of 0 if GTK_OVERLAY_SCROLLING is enabled (which is the default), so we
need to fix our overridden minimal height to never be greater than the
natural height.

Do that by only changing the minimal height if it is greater than what
we want.

I don't know why we didn't use to set 0 here, but it might be that a
height of 0 caused problems on older GTK versions, so keep 1 to be sure.

gdk_cursor_new_for_display.

Fill the scrollbars junction square with the appropriate style on
GTK version having a style for it (GTK >= 3.4).

Mostly fixes [bugs:#1611].

GtkScrolledWindow contains a bitfield, and GCC 3.4 and 4.8 don't agree
on the size of the structure (regardless of -mms-bitfields):

- GCC 3.4 has sizeof(GtkScrolledWindow)=88
- GCC 4.8 has sizeof(GtkScrolledWindow)=84

As Windows GTK2 bundle is built with GCC 3, it requires types derived
from GtkScrolledWindow to be at least 88 bytes, which means we need to
add some fake padding to fill in the extra 4 bytes.
There is however no other issue with the layout difference as we never
access any GtkScrolledWindow fields ourselves.

See http://lists.geany.org/pipermail/devel/2015-April/thread.html#9379