Reverse the default state for readonly property for help engine

The workaround introduced for helpEngine gives it write access to qhc file by default. This breaks existing installations in readonly locations. By reversing this default value to true it should prevent issues for this case.

Currently it's needed to add the following line:

helpEngineInstance->setProperty("_q_readonly", QVariant::fromValue<bool>(true));

to keep the help engine work properly in readonly location.

In addition we need to provide a public API for setting this value explicitly.

Not sure if it's not too late for 5.15.