Allow extensions to add extra meta item checkboxes to the page settings dialog

Like this:

ve.ui.MWSettingsPage.static.addMetaCheckbox(
    'metaItemName', // e.g. 'mwDisambiguation'
    'checkboxLabel' // e.g. 'Disambiguation page'
);

Depending on 'ext.visualEditor.mwmeta'

Bug: 61073
Change-Id: I78dd68ff5aad20c7e66c4ce1528b6f1bbc74892d
Merge "Allow extensions to add extra meta item checkboxes to the page settings dialog"