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The action that is taken when the user middle clicks on non client area
is configurable with gnome-tweak-tool. This allows users to disable any
action on middle click, lower windows, minimize windows, or toggle the
maximize state.

Also renames GConfTitlebarListener to GConfListener, since it's now
doing more than just checking maximize button order.

BUG=132061

Review URL: https://codereview.chromium.org/229783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262839 0039d316-1c4b-4281-b951-d872f2087c98