Provide own Toolbar, rather than toggling deprecated toolbar

The ToolbarItems in BookPage are no longer assigned to "tools", so
MainView won't mess with them.  Instead, we manually set the "tools"
property of the Toolbar that we explicitly include.  Toolbar.qml comes
from the UITK, with an import line added and one change to account for
the fact that the tools won't be changed.

Closes #60.
Fix initial enablement of forward and back buttons on BookPage

These should be set by the initial history.clear() call, but that's not
working on 14.04.  Hooking into the onOpenedChanged handler worked
around this when we were using the built-in toolbar, but that's not
working with our custom toolbar.  Instead, just set both to be disabled
at the beginning, because we know that to be the correct state.