common: Disable enum type stream operators for Qt >= 5.14

common: Disable enum type stream operators for Qt >= 5.14

Starting from version 5.14, Qt provides stream operators for enum
types, which collide with the ones we ship in types.h. Disable
Quassel's stream operators when compiling against Qt 5.14 or later.

Add a unit test that ensures that enum serialization honors the width
of the underlying type.