Several improvements to documentation

* List all applicable attached properties for each layout
* Move the stuff that describes min,pref and max sizes from GridLayout
  to the Layout docs.
* Document that all layouts \inherits Item. (Even if they don't directly
  inherit, this is consistent with how the positioners are documented)
* Add some links to Row, Column and Grid where appropriate
* Use \qmlattachedproperty instead of \qmlproperty for the attached
  properties. (Again, consistent with how the Positioners attached
  properties are documented)
* Change POSITIVE_INFINITE to the correct POSITIVE_INFINITY

Change-Id: Ia9272faa479b48a97300b031402c0380ca113d7b
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>