background: Fix memory corruption when creating preview

When using a single screen, the captured area was too small, and
we were copying data from out-of-bounds of the pixbuf area.

https://bugzilla.gnome.org/show_bug.cgi?id=696166
background: Remove unused "lock" preview widgets

https://bugzilla.gnome.org/show_bug.cgi?id=696166
background: Add support for lock screen background

https://bugzilla.gnome.org/show_bug.cgi?id=696166
shell: Use GDK_EVENT_* macros

Instead of TRUE/FALSE

https://bugzilla.gnome.org/show_bug.cgi?id=706268
shell: Start search when typing

https://bugzilla.gnome.org/show_bug.cgi?id=706268
shell: Remove setting invisible characters on entry

It's completely unused