icontheme: use g_file_load_contents() for symbolic icons

When loading a symbolic icon, g_file_get_contents() is currently used
with the icon pathname, to load its SVG data. This won't work when the
icon is not a local file, for instance when a symbolic icon is loaded
from a GFileIcon with a GResource path.
Fortunately GtkIconInfo already holds a GFile, so we can just use
g_file_load_contents() to load the data instead.

https://bugzilla.gnome.org/show_bug.cgi?id=709056
Fix typo in gtk-win32-base.css
Make it possible to hide custom titlebars

Previously, we were showing and hiding the custom titlebar
widget in response to state changes such as maximization.
Instead, use gtk_widget_set_child_visible() and leave
show/hide to applications. This makes it possible to set
a custom titlebar and hide it, for a titlebar-less appearance.

https://bugzilla.gnome.org/show_bug.cgi?id=707132
Improve gtk_container_add docs

Mention that some containers add intermediate children automatically.
action-namespace example: use non-deprecated function

g_simple_action_group_add_entries() is deprecated since we added the GActionMap
interface, so use its equivalent function instead.