Don't document Context2D's TextMetrics as its own QML type.

It can't be instantiated like a QML type, so it's actually better to
just document measureText() (which is the only way to construct objects
of this type) as returning an object containing some metrics for the
current font. This should have no effect on user code.

This solves the issue of the new TextMetrics type's documentation not
showing up. TextMetrics is a type that *can* be instantiated and is
more feature complete. It provides a declarative API for the functions
that take arguments in QFontMetrics.

Change-Id: I46251e6f3aa7179ab569f5131307181f71043df4
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>