This updates the behaviour of Typeface::getStringWidth,
Typeface::getGlyphPositions, and Typeface::getEdgeTableForGlyph to match
the documented behaviour of these functions. Previously, these functions
returned results normalised to a size of 1 point. The new (documented)
behaviour is to normalise to a JUCE height of 1.0 - that is, scaled so
that the sum of the ascent and descent is equal to 1.0.
The old API only allowed cancelling property "get" inquiries and
subscription updates. However, there are use-cases for cancelling other
requests too. e.g. switching between views in a JUCE app might mean that
it's no longer necessary to subscribe to a particular property.
Cancelling subscriptions ends up being quite involved. Different
handling is needed depending on whether the subscription is cancelled
before or after the responder replies to the initial request.
In addition, the responder may ask the initiator to retry a subscription
begin request.
