reduce priority of constraints when hiding and restore it when unhiding

This enables to hide either A or B in the following layout while C stays in position:

+-----+     +-----+
|  A  |-(0)-|  B  |
+-----+     +-----+
   |(1)        |(2)
+-----------------+
|        C        |
+-----------------+

(1) has A as its PWHidingMasterView, (2) has B. Both have the same priority originally.