Add settings for titlebar actions
Add settings for titlebar actions

These match the GSettings that mutter/metacity/gnome-shell have
for this. We change the default for the middle-click action to
none, since lower is just a terrible default.

We're slightly bending the rules here, since we're adding new
settings after .0, but a) it is just barely after .0, and b) settings
are not really application API.

https://bugzilla.gnome.org/show_bug.cgi?id=729782

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