Let users set __NOTOC__ & __FORCETOC__ status

This creates a new panel in the meta dialog, "Page settings", where page-
level settings will be, err, set. For now, this exposes just the
behavioural switches for the presence/absence of the Table Of Contents –
__NOTOC__ and __FORCETOC__.

As part of this change, the meta dialog is renamed to "Options" to be
less confusing, and the icon for the meta dialog is changed to the
generic one for dialogs, which was previously unused. The page settings
pane is provided first in this list, given that the categories pane
(amongst others) is now directly accessible through the toolbar menu.

Bug: 56866
Bug: 56867
Change-Id: I33ce05c19d2e19b249e1cefd26fd0e3697d0085d