load-graph.cpp: Draw the background explicitly (bgo #720560)

The gdk_window_set_background_pattern() function is a little dubious
to use unless you are writing GTK+ code since the background may be
overridden by gtkwidget.c whenever the style of the widget changes.

So instead of relying on this function, just paint the cached pattern
on every expose.
This fixes

https://bugzilla.redhat.com/show_bug.cgi?id=1015507

in which the background grid would disappear when the
gnome-system-monitor window was unfocused, then focused.

v2: Instead of caching a surface and creating a pattern for that
surface on each expose, keep the pattern in the cache
instead. (Requested by Robert Roth in
https://bugzilla.gnome.org/show_bug.cgi?id=720560).
Fix the size of the buttons in the headerbar

https://bugzilla.gnome.org/show_bug.cgi?id=721706
Fix headerbar item positions

https://bugzilla.gnome.org/show_bug.cgi?id=721734
Release 3.11.4
Fixed copy-paste mistake in release info in NEWS
Do not create and destroy unnecessary cairo_t
Use cairo_surface instead of cairo_pattern
Use auth_admin_keep in policykit permission file.

With auth_admin used the user had to enter the password once for
each process to kill/renice if performing an action on multiple
processes of other users. This way policykit remembers the auth
details for a brief period, so the user has to enter the password
only once.
Fixed graphic glitches when using stacked area chart
Do not clear the load graph when switching to/from stacked
Updated Norwegian bokmål translation
Fix URL of appdata screenshots and homepage after wiki reorganization
Added Scottish Gaelic translation
Updated FSF's address

https://bugzilla.gnome.org/show_bug.cgi?id=723226
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Updated Brazilian Portuguese translation
Release 3.11.5
New stubs for Mallard user docs
First draft: net-bits, solaris, units, priority
Wrote commandline.page, linked it to index