Add bottom pane toolbar option

[fossfreedom]

New in RB 2.99 and later is the ability to display a bottom pane.

Lets support this - simple enough tweak to do - will also fit nicely with the spectrum plugin I'm doing elsewhere.

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[fossfreedom]

https://dl.dropboxusercontent.com/u/3575524/bottom_toolbar.patch

patch for this - will put this on hold pending this bug report:

https://bugzilla.gnome.org/show_bug.cgi?id=731600